There are certain reference books I would grab quickly if my house was burning! Besides one very good dictionary, the following are my favorites on my bookshelves:
The Synonym Finder J. I. Rodale
Thesaurus of Alternatives to Worn-Out Words and Phrases by Robert Hartwell Fiske
The Persons, Places, and Things Spelling Dictionary by William C. Paxson (I STILL can’t spell the name of our California “governator” . . .)
The Time Life Series (found in most libraries . . . 1900-1910, 1910-1920, etc.)
Any Oxford Dictionary on various any theme is bound to be great.
If these books can’t be found any longer at your neighborhood independent bookstore, I know many of them can be purchased cheaply USED on Amazon or another used bookstore site. (I LOVE to recycle books, and my favorite place to frequent is any good used bookstore.)
What about YOU? What nonfiction helpful book do you frequently turn to during your writing work?