I've sold over a 100 books so far, my only regret is how many I still have left to get rid of.
Seriously, I play every week and I still own way more books than I have any sensible use for, cut it back to what you play and then play that, it's more fun and maybe some other guy can use the book you weren't.
Take The Riddle of Steel, I didn't take to it so I gave my copy to Imperator, who has run it and had fun with it. To me, that's a result, I no longer have a book I didn't take to taking up space and someone else is having fun with it. Everyone's a winner.
Sell and don't look back. Own what you play, collecting is bad for the hobby anyway.