Books for on iPad - What works?

Folks - Could anyone please advise me as to what eBooks work on the iPad? Here's a list of ebook providers from the Flagler County website. Are there others? THANKS!!!

• Google E-Bookstore - Free ebook selections compatible with many devices
• Open Library - Over 1,000,000 Free eBooks & more added all the time!
• InkMesh - A search engine that searches more than thirty online sources for free e-books
• Baen Books - Over 120 Science fiction and fantasy titles available, with new titles added every month.
• Internet Archive - Films, books, music & texts in many downloadable formats
• Project Gutenberg - Project Gutenberg makes ebooks and e-audiobooks available for some of the same great literature available in plain text.
• ManyBooks - Includes Project Gutenberg titles and beyond.
• Feedbooks - "A place to discover and publish ebooks"
• LibriVox E-audiobooks in the Public Domain
• Literal Systems -"Audiobooks you just can't buy"
• The Online Books Page University of Pennsylvania offers an index of hundreds of thousands of online books freely readable
• EBook Fling - Swap ebooks for free with other readers nationwide while still retaining ownership!
• The Spoken Alexandria Project - free Creative Commons library of spoken word recordings, consisting of classics in the public domain and modern works.