Many people are unhappy with the resolution of illustrations in e-books, particularly maps. This gets mentioned in a number of threads. I think if reviews mention illustration quality and book readers base their buying decisions in part on illustration quality and let retailers and publishers know about it, that overall illustration quality will improve. It would be good have reference images to serve as examples. I will start this off will some suggestions below, fully expecting these to eventually be replaced by better examples suggested by others, at every quality level.
There are a number of ways to improve illustration quality in e-books. Mainly, they need to be larger, clearer, and better encoded. Line drawings and maps should be PNG instead of JPEG, yet most such drawings that I come across are JPEG. JPEG is bad for these types of images in 2 important ways. JPEG is designed for photographic images, and is excellent for this purpose. But JPEG line drawing have visible artifacts and require quite a bit more space than PNG or GIF. When e-books are created from older works, the illustrations are often scanned from a paper copy of the book. In some of those it is possible to detect the text on the other side of the page, and even the next page after. Another problem is that some platforms need better support for image zooming.
This version of 'War and Peace' is free at Amazon at the moment and has several maps that look decent on my Kindles, but are not very demanding in detail and I have not studied much yet.
http://www.amazon.com/gp/product/B00JCDK5ME
See also this thread:
https://www.mobileread.com/forums/sho...d.php?t=246114
The Amazon free sample of The Heroes by Abercrombie has a map, see this thread:
https://www.mobileread.com/forums/sho...=242848&page=2
'Seven Pillars of Wisdom' by T.E. Lawrence
http://gutenberg.net.au/ebooks01/0100111h.html
has 4 detailed maps and several line drawings. These are small even for 600x800 pixel displays and suffer from the text on other side of page and next page problems and JPEG artifacts. These images look "OK" on the web, unless you are trying to read the smaller text on the Maps. On 166 DPI and higher e-readers, the text is even harder to read.
There are 4 e-book versions of this book on Amazon, only 2 seem to be illustrated based on the free samples. One has a photograph of one of the maps. The other (free sample) has 3 of the maps. They seem to have been lifted from the Project Gutenberg of Australia site then contrast enhanced.
The free Kindle sample for 'Seven Pillars of Wisdom: A Beautifully ...' is at:
http://www.amazon.com/Seven-Pillars-...dp/B00CONFDTY/
I now think its map image is a color scan oddly split into 2 images, left and right. None of the line drawings are in the free sample portion.
The free Kindle sample for 'Seven Pillars of Wisdom [Illustrated with ...' is at:
http://www.amazon.com/Seven-Pillars-...dp/B004XMQ6J0/
Three of the maps and none of the line drawings are in the free sample. This is the edition that I think the images were lifted from the PG Aus site.