I have (literally) two yards of books I haven’t read yet. (I get them at my local dollar store. I just counted, and there are 43 books in two piles, almost all of them hardbacks, that I got for a buck apiece.)
I also have a thing for unusual reference books. My collection includes such useful tomes as The Morris Dictionary of Word and Phrase Origins, the Dictionary of Euphemisms and Doubletalk, the Bliss Dictionary of Foreign Words and Phrases in Current English Usage, and many more. My wife just added one to my collection at Xmas this year that I think will come in very handy for me as a writer:
Talk the Talk: The Slang of 65 American Subcultures, by Luc Reid.
Not quite A to Z, it has the lingo of groups from Anime and Manga Fans to Witches, Wiccans and Neo-Pagans. If I ever need to write a character who is (for example) a parapsychologist, this book should help me make her sound authentic.