One of my favourite books in all the world as a kid was the Unearthed Arcana. It was basically a whole bunch of additional rules, spells, magical tat and assorted other goodness for first edition Advanced Dungeons and Dragons.
Not only was it an awesome set of extra kewl rules that we used and abused at whim ... it was and remains my fondest smelling book.
Seriously, it smells great.
Ten years ago I lent my high school copy to someone. Even as I lent it—that friendship well on the wane—I knew I'd never see it again.
I got a PDF of it a while back—so I could adapt my player's first-edition mage character to our D&D3.5 Hybrid rule-set for our play-by-email game (going on near 10 years old I think)—but a PDF ain't the same. I love actual rulebooks. Though I also appreciate the e-versions because you can search for terms and find a rule lickety-split. Not for me the anal indexing of my book with flags, neatly written on which,in fine penmanship, is what rule-treasure to be had in them thar page.
So it was a total bonus to get it at this awesome little bookshop friends told us about. Furthermore, since Unearthed Arcana had a bunch of errata, the rules updated in a later edition of Dragon magazine, the previous owner had in fact carefully sliced up the pages of those rules and meticulously spliced them into his book and over the offending error-ridden tables. Result? Book seller sold it for far less due to it being maimed ... even though it actually made the book better for it.
It was a joy get it back! I leafed through it at the shop's cafe as theBoy played tozzle on theWife's iPhone and theWife went for a scurry through the shelves.