Beware of poorly OCRd books, they plague the internet. Often when I search for an ebook I will find the same awfully OCRd version 5-20 times before locating a single version that is of exceptional quality. Sometimes you will be surprised and find that the epub/mobi ebooks are awful quality, and the .txt/.rtf/.doc files out there are quality corrected/OCR versions of texts (this is very much the case for older novels - 1970's and older prints).
Also, non-english texts and translations also have things that english versions leave out, such as pictures, chats, figures, etc. Try other formats that are less popular in english, shuch as FB2 and convert them. You will get better results.
This goes for all ebooks, especially ones with many versions/releases. Lots of the public domain ebooks released at the amazon store of are VERY LOW quality. You are often better off locating your own version for free and converting it from a higher quality source.
Ultimately convert all books to .mobi
For public domain books offering lots of formats:
http://www.feedbooks.com/ (high quality usually, have chapters/images, few errors)
http://www.archive.org/details/texts (varies, but there are lots of versions to preview)
http://www.gutenberg.org/wiki/Main_Page (most are free of errors, but not all have contents/image content)
As far as DRM is concerned, that is irritating. If you bought the book, I think you should be allowed to remove the DRM, especially if the DRM allows you to print the book and have a copy for yourself. I used to use virtual print driver to pdf and then acrobat ORC to accomplish this ages ago, but I bet there are more elegant ways today.