O'Reilly publishes a great book called Software Portability With Imake by Paul DuBois. Anyone using Imake extensively should have this one on their shelf. Again, this book is an absolute must if you need to customize the Imake configuration files for a different project.
The documentation that comes with X is quite good. Even the man page for imake has some useful information, unlike many man pages. A good place for quick reference is the Imake configuration files themselves. Typically, they reside in /usr/X11R6/lib/X11/config.
Information about make, in particular, is very useful. There are some very good GNU Info pages about GNU make. There are also plenty of books on the topic, and even the most basic Unix programming books include at least some information on using make.