They were there for us and then they weren't anymore and I for one still miss them, but Friends provides a pretty poor haul, book-wise.
There's the official book of the entire series, Friends... 'til the End: The One with All Ten Years, an Official Trivia Book (do you remember what Chandler did for a living?), an unofficial guide, Friends Like Us and finally The Very Best of Friends which is, I think, an official guide to the first three seasons.
And that's it! No novels, no prequels, no cultural/feminist studies. Very disappointing.