David Levithan's Boy Meets Boy is probably one of my all-time favourite gay books ever. Other books that represent the G in LGBT are The Vast Fields of Ordinary by Nick Burd, Love is the Higher Law by David Levithan, Will Grayson, Will Grayson by David Levithan and John Green, lots of others.
For the L, I love Ash by Malinda Lo (haven't read the sequel, Huntress, yet but I'm sure it's also awesome), Julie Anne Peters's books, and they're not YA but Sarah Waters writes amazing Victorian books about lesbian characters.
I can't think of any books about bisexual characters. Somebody able to fill in this gap?
For the T, Julie Anne Peters's Luna is really good, as well as Cris Beam's I Am J.