How many gossip girl books are there?and can you list the names in order??

There are 13 (including the prequel)

Gossip Girl
You Know You Love Me
All I Want Is Everything
Because I'm Worth It
I Like It Like That
You're the One That I Want
Nobody Does It Better
Nothing Can Keep Us Together
Only in Your Dreams
Would I Lie to You?
Don't You Forget About Me
I Will Always Love You (coming November 2009)

Prequel
Gossip Girl - It Had To Be You