"Look it up!"
I am sure, were you to ask either the Kid or the Kidette what phrase they heard thrown at them most often while growing up, I think "Look it up!" would top the list, even more than "Do your homework," "Clean your room" or "Eat your vegetables" -- I don't think we ever (or if we did it was rarely) used the phrase "What do you think you're doing?" because we just didn't want to hear an answer that was simultaneously silly and logical.
We would use an unfamiliar word, and they wanted to know whether or not they had been insulted -- look it up. We would mention an unfamiliar country -- look it up. A new writer -- look it up. Some ancient long-dead tribe -- look it up. Some archaic or foreign phrase -- look it up. Misspelled words in their English assignments -- look it up. The reason, of course, was because, if you look it up, you're much more likely to remember it; just giving an answer to a child instills laziness and a contempt for learning. The result is that we have two industrious, well-educated, smart aleck children with large vocabularies.
My favorite reference books are still books, you know the kind made with paper, and I use them all the time in my writing. My first go-to book is the dictionary, in my case the Funk & Wagnalls College Standard Dictionary, copyrighted 1943. In this world of spell-checks and digital illiteracy, you probably wonder why use a dictionary at all, much less one seventy years old: it comes from a time when accuracy mattered.
Part of the reason is to look up the spelling, and while we're on the subject, I should mention that nothing beats proofing your own work by reading it aloud -- a spell-checker won't help you if you've spelled "bog" as "boy" or "lost" as "lust," but you can surely hear the difference, and will save yourself much embarrassment. The main reason I use the dictionary is the obvious one, to look up the definition, to see if that word that popped so quickly into my mind is really the best one to carry the load I'm giving it. While the F&W is my workhorse lexicon, I have a few other dictionaries that I turn to from time to time. First, is the Britannica World Language Edition of Funk & Wagnalls Standard Dictionary (1959) in two volumes, mainly because of its seven-language dictionary. Another two-volume dictionary I use is The American College Encyclopedic Dictionary (1952), which I like because of the language commentaries in the first volume and the supplements in the second, as well as the clarity of the definitions throughout. Two other dictionaries used less often but still valuable are Webster's Third New International Dictionary and The Readers' Digest Great Encyclopedic Dictionary, which accompanied me when I left home long ago.
Another must-have book for any writer is a thesaurus, and I have two. My most used edition is Pocket Book's Roget's Pocket Thesaurus, which has been helping me to find the right word since the late 60s (and looks it). When I need a thesaurus with a wider range of selections I turn to Roget's International Thesaurus published by Crowell in 1962. Both of these books use the system developed by Dr Peter Mark Roget for his first thesaurus in 1852, that is words arranged by classification, then by divisions and sections, each step narrowing down groups of related words. I've tried using thesauri arranged alphabetically, but they just don't work for me. A thesaurus can be a very dangerous book in the wrong hands, as I found out when I was a newspaper editor in the 70s and encountered a young journalist who had been told in school not to use the word "said." Interviewees would shout, harangue, stump, blurt, exclaim, call, yell, whisper, murmur, repeat, state, aver...anything but "said." I could not get her to understand that words have shades of meaning, that words were generally not interchangeable, and everything depended upon context and connotation...so I locked her thesaurus in my desk drawer.
Another valuable book for a writer is neither dictionary nor thesaurus, but the Word Menu, created by the late Stephen Glazier. Its concept is stunningly simple -- divide the language into subjects, such as "Science and Technology" or "Institutions." Then break those subjects into smaller divisions, such as Mathematics or Politics. Then break them into even more specific lists. So, under "Part Three- Domestic Life" we have "The Home," under which have "Exterior Structure," a category of which is "Window, Walls & Facades," where we find short definitions for anta, bailey, bay window, bondstone, bull's-eye window (one of my favorite words), casement, Catherine wheel window...and on through wall, window and windowpane. To find the right word, you don't have to necessarily know what it means, just what, or where, it is. Stephen lived to see the completion of his monumental work but not to see it published. He died 20 January 1992, eight months before publication; he was 44.
What books you need as a writer, other than a good dictionary, a traditional thesaurus and a Word Menu, depends on what you write. I use an atlas quite often, but because most of my writing is historical I usually use atlases other people would consider outdated. Beside period atlases, I use several historical ones, particularly The Penguin Atlas of World History, as well as guidebooks from Baedeker and Muirhead. And because I also write of imaginary places I find that atlases of "maybe lands" are also handy. If you write suspense or spy novels, you might find The Ultimate Spy Book of use, and I find the Combat & Survival series in 28 volumes nifty as well. Fantasy writers might need books about dragons or the Middle Ages, or maybe A Field Guide to the Little People ("Marissa, do you still have my copy?"). Anyway, the kind of writer you are, the sort fiction or non-fiction you write, and your own curiosity level will determine which reference books you need to have at hand while you create...and you really do need books, especially in this digital age.