The Bactra Review Atlas of the
Murray R. Spiegel, Theory and Problems of Statistics, Schaum's
Outline Series (2nd ed., NY: McGraw-Hill, 1988). The formula is on p. 158 of
the first edition (NY: Schaum Publishing Co., 1961).
While the Schaum's book is a very convenient reference, it's not so easy
on the beginner; for that, try Larry Gonick and Woollcott Smith's Cartoon
Guide to Statistics (NY: HarperPerennial, 1993).