I expect that every writer has a shelf full of reference books to which they turn on a regular basis. Even a writer who works in fantasy, making up his or her setting and details as the work goes along, has to rely on a number of references for things like environmental information, literary techniques, poetic allusions, and the like. Some of my more frequently used reference works include the following.
:: The Rand McNally Atlas of the World: Masterpiece Edition. This is one of those gigantic atlases with close-up, topographical maps of just about every place on the earth. Immensely useful.
:: The Oxford Essential World Atlas. Why have two atlases? Because the Oxford one is smaller, being a large trade paperback as opposed to an eighteen-inch high, two-inch thick tome like the Rand-McNally. Sometimes I don't need the close-up view, so the Oxford suits me fine.
:: Historical Atlas, by William R. Shepherd. And then there are times when I need to know what the borders of the Ottoman Empire were, or perhaps the ecclesiastical districts of Plantagenet-era England, or the route of Marco Polo's journey to China, or the limits of Mongol encroachment into the West. This book has all that and more. I've meant to update this book over the years -- the one I own dates from 1956 -- but the old one hasn't lost its usefulness one whit.
:: Scientific American's How Things Work Today. I haven't had a whole lot of use for this book in my own writing yet, but it's full of information on the current state of technology for laypersons.
:: The New York Public Library Science Desk Reference. This is a big catch-all book about the current state of the sciences. If I ever turn to writing SF, I suppose I will use it more than I do now, although it's a fascinating book to dip into.
:: The Oxford Book of English Verse. Here's another one that I should probably update, as my copy is seventy years old. Every writer needs an all-purpose collection of poetry....
:: Immortal Poems, edited by Oscar Williams. ....or two. This one's more compact, being a mass-market paperback. It also contains a lot that the Oxford doesn't.
:: World Poetry, edited by Washburn, Major and Fadiman. This is just what the title says, and it's as wonderful as useful.
:: Merriam Webster's Encyclopedia of Literature. Also just what the title says. This book has capsule bios of authors, capsule entries on their major works, entries on literary techniques and movements, and a lot more.
:: The Bible, King James Version. I'm not much concerned with the theological aspects of the Bible, so I keep the KJV, which is a towering achievement of the language.
:: Almanacs. I'm not loyal to any particular almanac. I buy a new one every three years or so; currently I have the TIME Magazine almanac. I bought this one, as opposed to the World Almanac, simply because it was cheaper at the bookstore I was in on the day that I decided to update my almanac. You never know when you're going to need some tiny little fact like who won the 1949 World Series or when the Chrysler Building was erected or the dates of King Henry VII's reign in England.
:: The Elements of Style, by Strunk and White. This book is absolutely essential.
:: Dictionaries. Currently I own two: The American Heritage Dictionary, Second College Edition and The Oxford Pocket Dictionary and Thesaurus, American Edition. (I'm not sure how the Oxford can be remotely considered a "pocket" dictionary. The thing won't fit in any pocket I've ever seen.) My AHD has more entries than the Oxford, but the Oxford has the thesaurus in the same volume. I am also a member of a book club that includes access to the full Oxford English Dictionary online, although I don't use that perk as often as I originally thought I might.
Of course, there are a large number of other books that I use as reference materials, but the ones listed here are my main workhorses.