Hassan Aït-Kaci has written the excellent tutorial on the WAM entitled “Warren’s Abstract Machine: A Tutorial Reconstruction“. This book is out of print (I consider myself one of the lucky ones to have purchased a copy). For years it was available at vanx.org, but now the domain seems parked.
I had downloaded a copy of the files, and now the electronic version of the book has a new home at: http://wambook.sourceforge.net
Update 2013/04/13: Also available on https://github.com/a-yiorgos/wambook