If, like me, you use GNU Make a lot then you should be aware of two really important pieces of documentation for GNU Make that are totally free (speech and beer):
1. The GNU Make Manual. This is the standard manual that comes with GNU Make and is available on the web here: http://www.gnu.org/software/make/manual/make.html
2. Robert Mecklenburg's "Managing Projects with GNU Make". This is the book published by O'Reilly but released under the Free Documentation License. PDFs of each of the sections are available here: http://www.oreilly.com/catalog/make3/book/index.csp
It's great that these two resources are freely available, but don't let that stop you buying them. Supporting the FSF and Mecklenburg with a little cash is a good way of keeping free documents free.