As have J August Richards, Sam Anderson, Brigid Branagh (Wesleys girlfriend from Angel season 2), Andy Umberger (D'Hoffryn) and Eric Balfour (Jesse) although he seems to have been in pretty much everything.
by Incandenza
On a related note, just thought I'd mention that Harry Groener has appeared in The West Wing in the hope it might encourage a few more viewers.
As for the book: why?