|Photo from Vintagellu's Photostream on Flickr|
Mark Ryan had some help from at least four others in putting a little cash in Mr. Silverberg's pockets because Silverberg also wrote in this style under the names: Don Elliott, Loren Beauchamp, David Challon, and Gordon Mitchell. The books were published by Bediside, Nightstand, and Bedstand (names so similar that I'm guessing they might really all have been the same company).
I have not attempted to count the books that Silverberg wrote under these names, but there appears to be at least a couple of dozen of them, probably a good many more. By the way, a copy of Streets of Sin in excellent condition could be had five years ago for about $60.