Any of the editions of the Chemical Rubber Company Handbook of Chemistry and Physics; there are other CRC handbooks, such as the CRC Standard Mathematical Tables and Formulae, but “the” CRC handbook is the chemistry and physics reference. It is massive tome full of mathematical tables, physical constants of thousands of alloys and chemical compounds, dielectric strengths, vapor pressure, resistivity, and the like. Hackers have remarkably little actual use for these sorts of arcana, but are such information junkies that a large percentage of them acquire copies anyway and would feel vaguely bereft if they couldn't look up the magnetic susceptibility of potassium permanganate at a moment's notice. On hackers' bookshelves, the CRC handbook is rather likely to keep company with an unabridged Oxford English Dictionary and a good atlas.