GNU Autoconf, Automake and Libtool by Gary V. Vaughan, Ben Elliston, Tom Tromey and Ian Lance Taylor. Also known as "The Goat Book".
An Autotools Tutorial by A. Duret-Lutz. This is a great tutorial, but one of the nicest aspects of this web site is the list of Autotools resources at the bottom of the first page.
The GNU M4 Macro Processor Manual
Bruce Barnett's very well-written Unix utility tutorials
The AC Autoconf Macro Archive - an archive of drop-in components for autoconfiscated projects. Macros are fully classified for easy lookup, and full documentation for each macro is provided.