One of my favorites, the first (1967) edition of Ivan T. Sanderson's Uninvited Visitors and with a 3-D cover. Pretty good condition, too!
Over the years I've taken a lot of interest in all the various files the FBI have declassified and placed on their website, The Vault. Here's a few recently uploaded files:
And, the FBI's full file on racist occultist William Dudley Pelley. The file makes it very clear about his scummy, nutty, dangerous belief systems. It runs to more than 700 pages and is in 2 PDF files.