Today there are several copies of "The Knowlton Papers" on Amazon.com. They are either starting at $194 or are all going for $194 each. If you don't have a copy, you may want to consider the $194.
There are also two copies of "Study in Conjecture" on Amazon. One, if not both, are like $74. Myself I'm passing on this one. Eugene Hosey's review in a past issue of 'The Hatchet' - "On the Cutting Room Floor" - dissected this book, and it didn't sound like one I'd like.
It's not cheap, but I've found it hard to obtain the Knowlton Papers. I purchased the $194 copy (the others were all higher) and am waiting for it to be delivered.
I've only gotten through the first 164 pages and I've already picked up so much information that was new to me...and I've already got a lot of new ideas running through my head.