I don't know why I was surprised by this. I really shouldn't have been.

Gossip Girl, The Clique and The A-List are published by the same company that brought us the Sweet Valley High series:

Alloy Entertainment operates more like the romance novel industry than a traditional trade publisher. It has a staff in New York of about 10 editors who diligently research what's hot in the teen world  - what girls are wearing, the music they like, the TV shows they Tivo.

The book concept is created by Alloy.  Then they hire a writer like Cecily von Ziegesar:

She agrees that it may seem odd to some that she wasn't the inventor of the series. "People don't really get it _ it's such an unusual way to create books. But I'm very much the writer of my books, the final product is very much mine," she said. Churning out best-sellers for teens can get a little exhausting, she acknowledged, and it's good to have other creative minds at work.

So Cecily von Ziegesar, et al, are basically the Menudo of the book world.  Like I said.  I shouldn't have been surprised.