Half Price Books in central Austin has expanded the amounts of ephemera it deals in, mostly small, hard-to-categorize pamphlets. Many are in the 8.5"X5.5" saddle-stapled format of OD&D, a small-press genre in itself. Many are tourist books to archeological sites which could practically function as complete RPG modules:

Syria, Land History Tourism by Dr. Yussof Samara; Published by Atlas Bookshop, Damascus; No date -the map shows a 'Palestine,' but somehow I don't think this is an indication the book is pre-1948.
A thorough rundown of a few hundred sites, with history & legend. It could be played as a campaign world where all of these things exist at the same time. Or a very well-informed Co'Cthulhu game.

Demons and Monsters Tzetal Tales by Brian Stross; University of Missouri-Columbia; 1978.
Transcriptions & translations of Chiapas Mexican indian accounts. Some well-known like Backwardsfoot; most are possibly unique. Lots of good adventure-hooks nevertheless.
Particularly interesting are the legends of the conquering Spaniards, an outsider's depiction of Europeans. Watch out for Spaniards, because when they build a bridge or building they need a human sacrifice to bury in the foundations. Not entirely surprising, that was a widespread European practice in ancient times which is thought to have survived in modern folklore, such as the widow's son in Freemasonry. But what's it doing showing up with the Spanish colonials? The story is of such an off-beat provence, it begs to be turned into an RPG scenario.

Pirates and Buried Treasure by Jack Beater; Great Outdoors Publishing; 1981. An OCE-format booklet of Florida pirates stories. Filled with lurid details that probably don't show up as much in all-ages books.

Baedeker's Egypt by Baedeker Stuttgart; Prentice Hall; 1986. A hefty tourist guide notable for its precisely-drawn architectural maps, perfect for use as dungeons.

Diagnostic and Statistical Manual of Mental Disorders II, written & published by American Psychiatric Association; 1968.
A digest-size booklet with a paragraph describing each illness, great already for Co'Cthulhu [written in obsolete terminology]. The best part is the indexing system, numbered 1-1000, the same system used by all doctors on reports for all illnesses. The non-mental ailments are listed as well [although without description], all of which is easily rolled with D10s. Some choice critical hits for your PCs to suffer:
020 Plague
129 Intestinal parasitism, unspecified
297 Paranoid states
607 Other diseases of male genital organs
680 Boli and carbuncle
800 Fracture of vault of skull
801 Fracture of base of skull
804 Multiple fractures involving skull or face with other bones
870 Open wound of eye and orbit
871 Enucleation of eye
931 Foreign body in ear
932 Foreign body in nose
990 Effects of radiation
E842 Fall in, on, or from airplane
E883 Fall into hole or other opening in surface
E908 Cataclysm
E961 Assault by corrosive or caustic substances, except poisoning
E962 Assault by poisoning
E978 Legal execution
E990 Injury due to war operations by fires and conflagrations
E996 Injury due to war operations by nuclear weapons

The rare books room itself is filled with unreasonable & inexplicable prices. In the rest of the store, a pretty huge space [former supermarket] for a used book store, more ephemera/booklets/oddball publications may be found. I try to buy as much as possible from the clearance section, which has received stuff from the RPG section lately.