The free books, of course, are books in public domain. So the "classics" are mainly what you are going to get. Along with my suggestions in my earlier post there are thousands of ebooks at the Gutenberg site.
http://www.gutenberg.org/wiki/Main_Page . Most are "lit" files that are easy to convert with Calibre. To me the 150 "free books" is a non-issue. 10 minutes on the internet and you can access those 150, plus thousands more.
Now that you have the reader up and running..... enjoy!
