MetaFiler has a cute thread going (link) on the question “what is the most useful book you own?” Only one vote so far for The Bible, one for The Torah, same for Gravity’s Rainbow, none for the Complete Shakespeare. Lots of cookbooks, mostly Joy of Cooking (eeuw), but a couple of votes for stuff from Cook’s Illustrated (ah, much better). One for Handbook of Poisoning, a medical text. One for Kenkyusha’s New Japanese-English Dictionary (5th ed.), one for Peter Kennedy’s A Guide to Econometrics, one for A Uniform System of Citation (18th ed.).
So it takes all kinds. But I go with the guy who stretched the definition somewhat and declared the most useful book he owned to be Wikipedia. Copy that, I go there 20 times a day.