I have a few to add, mostly in the contest mathematics and problem books section.
I'm a bit worn out after ARML, though (I was head coach for the Northern California teams: our A team came in first in the west, fourth overall; our B team came in third in the west B division), so give me a day or two to put my list together. Still, I want to point out
Publisher's of ARML/NYSML contest problem book for 1989-1994. the Leningrad Math Olympiad problem book 1987-1991 Index to Mathematical Problems 1980-1984 [a REMARKABLE collection] Problems and Solutions from "The Mathematical Visitor" 1877-1894.
2) Greater Testing Concepts (new address: PO Box 5014, New York, NY 10185-5014) Richard Rusczyk's and Sandor Lehoczky's self-published, two volume "Art of Problem Solving"
(I don't have a phone number for them, but you can probably email richard at firstname.lastname@example.org)