My essential books are:
"English Through the Ages" by William Brohaugh. From Old English to modern-day slang, a word-by-word birth record of thousands of interesting words. In use in 1605: tumble, verb, as in to have sex with. Used as a noun, 1905. Fascinating to read, if you're a word lover, and useful for writing historical romances.
"The Joy of Cooking" by Irma S. Rombauer and Marion Rombauer Becker. THE basic cookbook to have.
"Planets in Transit" by Robert Hand. The best astrology book ever written. (Transits are aspects between planets in real time and one's natal horoscope. They describe the energies one experiences, positive or negative. Mr. Hand suggests how best to benefit, or survive, them.)
"Flip Dictionary" by Barbara Ann Kipfer. For when you know what you want to say but can't think of the word. A super thesaurus.
"Stella Natura Biodynamic Planting Calendar." This isn't a book, but I open it more often than any other collation of stapled pages in any given year. Month-by-month, day-by-day charts of moon cycles. I plant seeds by the moon to ensure crop success, and would hate to have to decipher the US Naval Observatory data myself.