The necronomicon, closely followed by the 9 Gates. I picked up a copy of Al Azif, the Necronomicon, a few years ago; it's a hardcover reprint of the spoof version produced by L. Sprague De Camp and Owlswick Press in the early 1970's.
The 9 Gates I purchased from Mike Haspil - spectacular work. The accomanying artifacts were exquisitely detailed.