I won a Barnes & Nobles gift card at the white elephant gift exchange last Christmas at my grandparents-in-law's. Rediscovering it recently, I have been poking around on B&N trying to figure out how to best spend my $20.
I long since discovered the wonders of buying books for $.01 + S/H from Amazon, using this amazing resource to fill in many a hole in my sci-fi / fantasy library. When my friend lent me Halbritter's Armory recently and proudly told me I "wouldn't find another copy" of this "rare" and out-of-print book I didn't say anything, but sure enough, I found it for one penny on Amazon.
My current working list of B&N titles that I am trying to choose from (after a very brief session of poking around in the B&N search engine):
"Weapons - an International Encyclopedia from 5000 B.C. to 2000 A.D." for $.09 + S/H;
"Encyclopedia of the World's Special Forces" for $.21 + S/H;
"The Encyclopedia of Genetic Disorders and Birth Defects" for $.12 + S/H;
"Ancient Wisdom and Secret Sects" for $.01 + S/H;
"A Dictionary of the Martial Arts" for $.01 + S/H
"The Encyclopedia of Celtic Wisdom: A Celtic Shamans Sourcebook" for $.01 + S/H
"The Encyclopedia of Witches and Witchcraft" for $.85 + S/H
"The Ultimate Dinosaur Book" $.34 + S/H
"A Bestiary" $.01 +S/H
The question begging to be asked is why in the world anyone would spend money on buying products created for RPGs when products created for other markets with much higher production values are available for pennies and are infinitely more useful than all the RPG crap ever put up for sale.
I have a large collection of RPG books that I never use. I compulsively buy cool RPG books when I encounter them in the wild, sometimes irrespective of the fact that I own the exact same edition of that particular book. The desire that I have to buy cool looking RPG books when I encounter them in used bookstores and comic shops has nothing to do with their utility in my games. I will not use them. Most of the material sprung upon my players in any campaign I run is original Carl madness. I do use some published resources, however, and they are hardly ever "RPG Products".
I do use my ethnographies from my days as a cultural anthropology major at the University of Oregon quite often in my campaigns, and academic texts on subjects as disparate as the city states and orthography of ancient Sumeria and quantum mechanics.
Imagine a subject that is of paramount importance in the RPG you are currently playing in. Google that subject with possible book titles in mind and figure out what kind of books are out there on that subject. Search for those books on Amazon or B&N or just freakin' Google it again or something. If you do a rational analysis of the bang for the buck it makes no sense to buy an RPG product in physical form for $10-15 + S/H when you can get multiple books of source material on the same subject for less money.
I'm not sayin' I'm just saying.