My latest acquisition. I finally gave in, bit the bullet and purchased this unique piece of roleplaying history. While it is a wargame, it really sits in that weird space between rpg and wargame that so many of these games did in the mid 70's. It does include rules for 1:1 combat, and used in conjunction with the LBB's you could easily make this into an RPG, in that same spirit of individualism that early D&D had.
If you don't know the history of this book it is really quite interesting. Gary and Brian put this together without permission from the Burroughs estate, and were almost immediately told to cease printing and selling these books when it was published. So I don't think there are a whole lot of copies out there.
John Carter, Warlord of Mars Adventure Gaming Handbook by Heritage Models Inc. (though, thankfully, it doesn't have Princess Points!)
I've had a PDF of this book for some time, but the actual book is much better than I thought it would be. By today's standards it is not particularly well written, organized, or illustrated - but it certainly has charm. I'm hoping that next year's John Carter movie will be wildly successful which might make this book just a little bit more valuable. :)