Of course you know that out of print isn't the same as out of copyright; the Gutenberg collection is really old. There are some great classics in there. I've been reading books from Gutenberg since my first PDA in 1999, and still have a lot i'd like to read.

Mark Twain's A Connecticut Yankee in King Arthur's Court was fun, along with Huck and Tom's adventures. HG Wells, Dickens, Tolstoy. I think Ayn Rand's stuff is there now, too. Tolkein...

One great thing about FBReader-like "kitchen sink" readers is that they support plucker format. A lot of my ebook collection is stuff i grabbed ("plucked") from online publications using a plucker tool. It packages up a series of web pages into a single file for reading, and works surprisingly well. This isn't as useful as it used to be before wifi and 3G smartphones, but it still beats trying to read a novel in the browser a chapter at a time.