Clearly passionate about math, Weisstein, an astronomer, began compiling this one-man labor-of-love reference about 14 years ago during his college days. Topics reflect the author's interests, as he says, his "own random walk through mathematics," rather than a fixed set of criteria. Differing from other math dictionaries, this one contains extensive cross referencing to related entries as well as internet sites; the entries contain both standard and popular references; and the abundant examples include explicit formulas and derivations. Weisstein has added about 3,600 new entries to this second edition (which is about one-third larger than the first, published in 1999), updated everything (including bibliographies), and enhanced many entries by incorporating Mathematica commands (for performing calculations and producing graphical displays and illustrations). At more than 3,000 pages (necessarily thin paper, to fit in one volume), this work is a unique reference and a terrific value pricewise.