It turns out there are! The author's next book is (in Star Wars style) the first in a trilogy of prequels to The Mortal Instrument series. The next series is called The Infernal Devices and the first one comes out Aug 31st 2010. Right after that there's a jump forward again and the fourth book in the Mortal Instruments series will be released in Spring of 2011.
So we've had books 4, 5, 6. Next is book 1 then book 7. That's how I read it anyway. Correct me if I"m wrong... it's a tough time line
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I had some issues with the series (mostly characters acting out of character and making odd choices/decisions or being brilliant one minute and then so stupid the next that they don't catch something very obvious the next) but that didn't stop me from gobbling up the books. The action is well paced and the "lingo" feels organic. I like the writing style.