There is a wealth of info on this and related subjects on the comicart-l group. When I first discovered the group a year or so ago I spent quite a while browsing through old posts. There's not one post or group of posts I can point you to because it's scattered far and wide throughout that board's history, but if you start browsing through old posts you can come away with some idea of what the art collecting community considers to be probable regarding the existence (and/or current disposition in the marketplace) of important silver Marvel art.
Just to throw out an example related to the topic at hand, I believe I saw one old thread detailing which Ditko ASM books had been broken up, whether pages had been on the market yet, etc.
You've got to work your way through lots of posts and search around for artists/books that interest you, but for those who are interested in the mysteries of silver Marvel art, spending some time searching through that list is pretty much required reading. As far as I know, it's the best public source of info on the topic.
http://groups.yahoo.com/group/comicart-l/