I used to collect reference books but I find I rarely use them anymore. It's just so much more convenient to look things up on the Internet.

Back when I did use reference books frequently, the one I used most often was The Columbia Encyclopedia. It's a one-volume encyclopedia, but as big and thick as an unabridged dictionary. Not to be confused with the Concise Columbia Encyclopedia, or the Columbia-Viking Desk Encyclopedia, which are (or were) much smaller abridged versions of the Columbia Encyclopedia, but still very useful. I used to give paperback copies of the CVDE to students I knew.

For browsing, I loved almanacs. The World Almanac, The New York Times Almanac, The Information Please Almanac (do they still publish that one?). I used to buy a different brand every year to minimize redundancy. Somewhere along the line I also picked up copies of Whitaker's Almanack and Pears Cyclopaedia, which give a different perspective.

Then there are my bird books, and my wildflower and other flora and fauna identification guides. I do still use those, because when you don't know what something is called, it's much easier to flip through the pictures in a real book until you find a match, than to try something similar on the Internet. (Unless there's a web site out there that I haven't found yet, that helps you with this sort of thing.)

And there's the Reader's Digest Complete Do-It-Yourself Manual and ...Fix-It-Yourself Manual, which are very good, but I have only dipped into them when I can no longer avoid it!