I just found a web site with some information on him - I have one of his books and admire his work, so your question caught my eye =). Here are the leading clients:
Shelley Bates, William Bernhardt, Anne Bishop, Jim Butcher, Daniel Depp, Diane Duane, Jane Haddam, Nalo Hopkinson, Stuart Kaminsky, Richard Knaak, Kay Kenyon, Chris Knopf, Jay Lake, Todd McCaffrey, Robert McCammon, Anne Perry, Cherie Priest, Ken Scholes, Karl Schroeder, Brent Weeks.
Here's the link to check it out yourself:
Hope this helps!