Theatre has The Backstage Handbook; the rest of the world has Pocket Ref. If you're smart, you'll have both of them. Pocket Ref is even smaller than The Backstage Handbook, but packed with probably even more information. A few random examples include: the formula for air velocity in a pipe; densities of hardwoods; maximum floor joist spans; Epson and IBM printer codes; mathematical constants; US holidays; CB codes; telephone area codes; names of international airports; types of paints and glues; water pollutant standards; tempering colour for steel; Roman numerals from 1 to 100,000. Need to know the manufacturer code for Alliance Tires from Israel? the radioisotope half-life of Bi210? the actual size of Quad Demi paper? the two-letter code of Royal Brunei Airlines? the maximum slope for a conveyor belt carrying loose cement? the currency of Laos? how many litres in a British firkin? how about a US firkin? the weight of 20 cubic feet of flaked animal glue? It's all in here and more - more than you'd ever imagine.
All this in a book a bit bigger than a pack of cigarettes. Sure the paper is onionskin-thin and the type is tiny, but neither gets in the way of its usefulness. Of course most of the information in this book is good only for answering trivia questions, but there are so many different areas of data covered here that it's almost certain to be useful to everyone eventually.
Pocket Ref was first published in 1989 and this is already the fourteenth printing of the first edition! The author seems to be intent on keeping it up-to-date and correct, which means there should be a new edition every few years. I bought my copy at Lee Valley Tools, but it is available at bookstores as well.