The Author's Webs
Note: The books in the series were all written by both of the authors together.
Holly Black: blackholly.com/
Cassandra Clare : www.cassandraclare.com/
The Magisterium series
The Iron Trial
The Copper Gauntlet
Other books by the authors
Holly Black also wrote the popular fantasy series called the "Spiderwick Chronicles" with Tony DiTerlizzi.
Cassandra Clare has also written teen/adult series such as the "Shadowhunters Chronicles".