A particularly great book at describing GNU Make is Managing Projects with GNU Make, which is published by O'Reilly freely under their Open Books Project. In fact, the book itself is distributed under the GNU Free Documentation License which allows me to redistribute an unmodified version of the documentation.
I took the liberty of assembling all of the individual chapters of this document into one cohesive pdf file with TOC links to individual chapters as well as bookmarks. While there are some particularly advanced topics that I could probably add a section on based on my extensive hacking of The Bionic C Library alone (e.g. the Android Build System, or Nonrecursive Automake), I have not modified the documentation in any way, I have just added meta-information to it*.
Some improvements I could see being made to the meta-information are below.
- inter-section references (i.e. links made to a different section from within the text of another)
- Index links (i.e. links to the exact page for each entry in the index)
If you are able to volunteer to make those improvements and share your changes, I would be happy to redistribute them here.
If you like this book, I encourage you to buy a hard copy, or if you feel like GNU Make has benefitted you enough, please donate to the Free Software Foundation.
*I have added the cover to that file as well, so please inform me if I have in any way violated the spirit of the license and I will remove it.