|
Malleus Monstrorum - Basically a large bestiary for CoC. One of the best bestiaries put out by any RPG game, in my opinion.
H.P. Lovecraft's Dreamlands - Covers the Dreamlands, if you plan on taking adventures beyond the veils of sleep. (Link is PDF version, dunno if there are any hardcovers still available anywhere.)
Beyond that, there's nothing really that I would consider essential. That's one of the strengths of CoC, in my opinion, that just the core rulebook is so amazingly inclusive.
Check this thread as it is filled with good info from people and their suggestions of adventures to run.