Lauren Myracle's TTYL series has rocked the bestseller lists (TTYL, TTFN and L8R G8R) and they better darn well be rocking your bookshelves. Apart from the fact that she co-wrote with E. Lockhart for How to Be Bad, her books are must-reads because... well, they're written in IM.
Some may see this as the apocalypse of literature, but it's not. Myracle manages to create 3-D characters without any prose or description, and make it seem natural. I love the complex friendships between Zoe, Maddie and Angela because they're real without being boring. The problems they face with boys, Queen Bees, faith, doubt, each other and themselves are not easily solved and won't be fixed without sacrifice.
Put them on your TBR.