Any material in your book that is borrowed from another source may require written permission. The goal is to distinguish between material that can be used without obtaining written permission and material for which such permission is necessary. These guidelines should help you decide the majority of cases; if in doubt; feel free to ask us.
Using material without the need to obtain permission is called "fair use." Your use of the material is considered "fair" to the original copyright holder. This is a marvelous privilege that can save you lots of time and work, but you need to use it carefully. Fair use is defined in terms of the proportion of the whole work being used. The Chicago Manual of Style ( Chicago University Press, 1982) tells us that
an author should not quote at such length from another source that he diminishes the value of that source . . . proportion is more important than the absolute length of a quotation: to quote five hundred words from an essay of five thousand is bound to be more serious than to quote the same number of words from a work of fifty thousand.