Hi all,
I'm a cross-platform fanatic. I regularly read e-books on my 3 iPads (all 3 common sizes), on my 4 Android tablets (4 various sizes & shapes from 8 to 14 inches), and on my Android phone. I process my highlights and annotations (I heavily annotate the books I read) on my Windows computers. (I used to be an iPhone and Mac user in the past, but I ditched them both years ago, Apple software being so insufferably dumb and limiting; I can just barely tolerate it on the 3 iPads, running in the background.)
So you can guess my struggles: what would be the best e-reader for me, given my everyday cross-platform use?
Brief answer: the situation is pretty dire. But there may be some hope on the horizon.
The best e-reader on Android, to me, is
Moon+ Reader Pro, hands down. It's far from perfect, so that I would rate it only 4 stars out of 5, but I'm afraid there's nothing better. I like
ReadEra and some others, but Moon is the best of the bunch. My issue, though, is that it's
Android-only, although it makes it possible to export annotations to the PC, in a crude and heavy-handed fashion.
The best e-reader on iPad/iPhone, to me, is
Marvin, although I like a few others, too, such as
MapleRead, but Marvin is still the best of them, I'd say, despite being
abandonware for years now, unfortunately.

Marvin's fundamental flaw is that it does not even offer annotations syncing among iOS devices (!), let alone cross-platform syncing. I would therefore rate it 3˝ out of 5 stars. But, at the very least, export of Marvin annotations to the PC is possible – once again, in a crude and heavy-handed fashion, just like in Moon.
(I remember that while Marvin was still in development, Kris, its developer, approached the Taiwanese gentleman, the Moon developer, to try and develop a
common cross-platform syncing solution between Marvin and Moon, which would be just fabulous, but nothing came out of it. I think there may be a severe language barrier – the Taiwanese developer, despite being a really nice and forthcoming person by all accounts, is not overly communicative when it comes to advanced feature requests like these.)
So, as you can see, I'm really struggling. Whenever I read an e-book, I need to export annotations from it
separately from Android and the iPads (separately for each of the 3 iPads, in fact, due to Marvin's failure to provide any syncing at all), and then I need to collate those annotations manually on a Windows PC. It's a hassle and a chore!
Now, you might ask: what about
Kindle or
Google Play Books? Don't they offer
exactly what you're asking for out of the box?
Nope, unfortunately. They're both too dumb, too crude, too elementary – just as you'd expect corporate software to be. Corporate software caters to the masses and sneers at power users who demand advanced functionality – especially
customizability. (See Apple software which is the epitome of despising power users and their customizability needs.)
Quite apart from that, Kindle and Play Books are simply
unusable for my needs, period.
Why? Because so far, we've only mentioned
EPUB books. Yet many books I need to read are only available as
PDF files. (Nope, automatic converters from PDF to EPUB
never work properly; you might just as well stay in PDF.) Worse still: some books are only available in
photographed (
scanned) PDF files, containing zero actual text (just
images of text; and again, automatic OCR conversion to text-based PDF files never works properly – you might just as well stay with the image-based PDF file). In fact, I often need to photograph a printed book manually if it's one that is otherwise unavailable – the result is an imaged-based PDF file, not a text-based PDF file.
And, in case you didn't know, neither Kindle nor Google Play Books support
any annotating of
image-based PDF files
at all.

The only way to annotate
image-based PDF files is via
freehand drawing, which neither Kindle nor Play Books support, and so, Kindle and Play Books can't be used by me, even if I were to tolerate how generally dumb and limiting they are.
There is some elementary and very crude support of freehand annotations of image-based PDF files in Moon+ Reader Pro on Android, but it's nothing to write home about. As to Marvin, it doesn't even support
opening PDF files.
And so, for reading those
image-based PDF files, I need to resort to a third piece of e-reader software:
Adobe Acrobat. Its cross-platform functionality, in terms of (not only) those freehand annotations, appears to be rock-solid.

Of course, it's
corporate software, so yes: it's obtuse, it's dumb, it's limiting, unintelligent, unwieldy and heavy-handed – what else can you expect from corporate software? But at least it
works reliably.
For many years, for reading PDF files, I used to prefer
GoodReader on iOS and
Xodo on Android, because unlike Adobe Acrobat, they are neither unwieldy nor dumb. However, both GoodReader and Xodo have
major issues with
syncing those freehand annotations, even leading to pretty frequent data loss (I use Dropbox for syncing), and so, regrettably, I had to ditch both GoodReader and Xodo.
So, is there really no
single cross-platform app that would satisfy all my e-reading needs?
Well, perhaps there's some hope on the horizon. I've recently become aware of the cross-platform e-reader
BookFusion, and I've been giving it a test drive on all 3 platforms (iPads, Android, web/Windows).
It's not "there" yet by any means (Moon+ Reader Pro is currently still clearly better on Android, and Marvin or MapleRead are currently still clearly better on iOS), but it certainly appears to be moving in the right direction!

For one thing, it has those lofty cross-platform ambitions that Moon Reader or Marvin never even aspired to.
The free plan in BookFusion is fairly limiting, and anyway, I can't possibly use BookFusion as my
only e-reader at this point. There are several showstoppers for me – the main being, just like in Kindle and Google Play Books, the
inability to annotate image-based PDF files via freehand drawing. But whereas I have
zero hope that Kindle or Play Books will ever implement this feature (they are
corporate software, after all, and so they
despise users who ask for
advanced features and
customizability; they couldn't care less about them, because power users always only constitute a
tiny proportion of their gigantic user base!), I do trust that it's possible for BookFusion to implement this crucial missing feature (and others) at a future point that is not all too distant.
I was surprised and delighted, however, that there is
one feature where BookFusion, even at this rather early point in its development, excels over all the other e-reader apps. So, in fact, I
do use BookFusion as my main e-reader (on both Android and iPads) for
one of the books I'm currently reading. (I used to say, "I always read 7 books simultaneously", but nowadays, it's truer to say "I always read at least 12 books simultaneously".)
You see, one of the 12+ books I'm currently reading is a
bilingual book. I purchased a printed copy of it years ago – but I stopped reading
all printed books way back in 2010 when Steve invented the iPads, and I've only been reading e-books since. So, I also got an electronic copy of this bilingual book (French & English – for learners of French, and I am one).
In the printed book, everything is 100% neat and clear: original French text on the left – translated English text on the right.
No matter which e-book app I initially tried, on
any platform – the
electronic version of this bilingual book is pretty much messed up. Still readable, but messed up and disorganized. No matter how much I experimented with adjusting the font size, double-column width, etc. – the result in the electronic copy of that book never was the same as in the printed version: French on the left, English on the right. Moon+ Reader Pro, Marvin, ReadEra, MapleRead, Kindle, Play Books (you name it...) – all have failed in this regard.
Well, but here comes BookFusion!

It seems like miracle, but BookFusion, as the
only e-reader app I tested, manages to display this bilingual book just as neatly in its electronic copy as in the printed version: it consistently displays French on the left, and English on the right. In Android, in iOS, in Windows (on the web). There must be some special coding within the e-book that all other e-reader apps choose to ignore, but BookFusion does respect. Here's a screenshot from BookFusion:
Before I found BookFusion, I was forced to resort to pretty desparate measures: using two various apps side-by-side (Moon & BookEra on Android; Marvin and its clone Marvin SxS on iPads), just to somehow manipulate the text so that I can see French on the left, English on the right.
See my
tweet from back in July on this. Among other things, the screenshots there prove that e-books do look a lot better in Moon+ Reader and in Marvin than they currently do in BookFusion... But in this instance,
functionality trumps looks for me (because as soon as you flip a page in Moon or Marvin, all the French & English gets mixed up – unlike in BookFusion), and so I switched to BookFusion for good, for reading this particular bilingual book.
As a token of my gratitude, I subscribed to BookFusion's lowest paid annual plan (around €19), even though I'll probably only be using BookFusion to read this particular bilingual book and nothing else,

which would be perfectly possible in the free version as well. In a year from now, I'll reconsider my subscription – if BookFusion makes substantial progress within the next 12 months, I'll be happy to prolong. I do believe developers should be rewarded for their efforts, and if the day comes when BookFusion satisfies
all my reading needs, I would subscribe to their top paid plan (currently $96 annually) without blinking an eye.
I think the BookFusion developer is lurking somewhere around these MobileRead forums, and I hope my feedback from this post will be useful for him. To make it even more useful, in a later post in this thread, I'll be specifically listing all the showstoppers but also lesser deficiencies that currently make it impossible for me to consider BookFusion my main (let alone my
only) e-reader.
If anyone reading this has other recommendations for quality cross-platform e-reader apps, do post them here!

Or, if you have any specific experience with BookFusion – any pitfalls there that I, as a novice user, may not have discovered at this point?