.epub is the best format to use, and FBreader supports it.

Project Gutenberg (www.gutenberg.org, NOT .com) is the best source of free ebooks and they put out new ones daily.

Another good format to consider for FBreader is the zipped .htm format. That works well for me.

There is enough fun and enlightenment in Gutenberg.org to last anyone several lifetimes. If you want more, for fairly low prices there are audiobooks from Audible.com or Kindle books from Amazon. You can read Kindle books for free on a PC and you don't need a Kindle. Audible and Amazon books are DRMed, which is a religious issue for some. And neither work on the N900 so far.