Well like I said, it is a matter of personal preference as far as library size on a device. I definitely see the allure of having most or all of your vast library on one or all of your devices. I used to do that. But now a days I'm content to keep the vast library stored on a couple of computers and various backup drives. I keep it organized by hand--I don't use any management apps like calibre. I then locate the books I want to put on a particular device and transfer them. The vast majority of books on my current devices are also housed in the Kindle cloud so they are easy to transfer and they automatically keep my bookmarks, notes, and current reading position updated on all the other devices. That was always a PITA back when I used calibre and modified books. It is so easy with the Kindle cloud though, but it only works with Kindle books bought at Amazon and unmodified, hence the reason I don't modify ebooks any more. So for me that is the best of both worlds. I have my complete, organized library on my computers safely stored away at home, and I have the books I need or want at the current moment on my portable devices. When I was young and in grad school I had a vast print book library and I can guarantee I only carried around a few of those at any one time. The vast majority stayed at home on my bookshelves. I've slowly returned to that concept, but in digital format now.
I remember spending many hours in the university library and as a grad student I even had a small office there that the university library provided for grad students which contained a small desk and a few shelves. It was only about the size of an old phone booth, but it was good for doing research in the library rather than having to lug books back and forth between library and home. I would have killed to have had the book storage capability we have now back then, although most of those books probably still aren't in digital format--not even PDF. But that was over 35 years ago, and now I no longer need to have research books with me 24x7x52! Now I can just read for pleasure. One of my favorite memories after I graduated university was that for the first time in years I could once again choose what I read and whether or not I finished it. I rarely had that luxury in university. It was SO wonderful to be able to pick up a book, decide it wasn't good, and then be able to put it down and never go back to it again. One of life's little pleasures now.
