Your favorite math reference book(s)?

[Accutron]

Practical Mathematics, edited by Reginald Stevens Kimball. There are many different editions, including single volume editions with about 1300 pages, a four volume set and a periodical format released in over a dozen issues, but I think the content is pretty much the same across all versions. The book covers many important topics in both academic and applied mathematics, and presumes that the reader is using nothing more elaborate than a slide rule or maybe a mechanical adder. Topics include algebra, plane and solid geometry, statistics, differential equations, calculus, accounting and finance, chemical equations, radio and electrical calculations, thermodynamics, structural engineering, gunnery calculations, log tables, and of course a section on how to operate a slide rule. I consider it an absolutely essential reference book.