A Book Test is a Mentalism routine in which words or pages in a book selected by a spectator is able to be divined by the performer.
Book Tests generally fall into three categories: those that can be done with any book, those that rely on a special Forcing Book and "hybrid"/"natural" forcing books which are done with a particular regular book.
Dick Christian is writing the "Encyclopedia of Forcing Books and Book Tests" which he has been compiling for years and hopes to have ready for publication by the end of 2011.
In From Witchcraft to Card Tricks (page 7) Stephen Minch writes that there is "evidence to show that book tests were being done as early as the fourth century". He mentions reports by St. Augustine and also earlier exposés by Lucian and Saint Hippolytus of methods for reading sealed messages and fraudulent mind reading.
Using a hymnal as a book test was mentioned in "The Freeman" [Indianapolis, IN]. June 8 1901, p 2 col 2.: "Prof. J. H. Moore, the Negro wizard, is noted for wonderful feats of legerdemain. His slate trick consists in answering questions with the slate and pencil inverted, writing hymns and giving number of page in hymn book when the hymn is found, the book being in the hand of the invited guest."