IMHO most of the "books everyone should read" are out-of-copyright classics, and available through PG, Manybooks, etc etc. We have a "Book Uploads" sub-section on this forum where you'll find a fair number of books that various people (and thanks to everyone who's contributed!) have created nice Reader-specific versions of.
The main source of "free" books is
Project Gutenberg. They have well over 20,000 books which can be downloaded free of charge.
Although you can use a memory card with the Reader, most people seem not to bother. You can store a very large number of books in the Reader's internal memory (100-ish, depending on their size) and, since the Reader only has a "flat" file system, there's really no point in storing 1000 books on a memory card if you have to page through 100 screens in order to find one
.