I found this downloadable buyer's guide from the UK had some good practical tips not found elsewher. I only used the 3.2 guide but there are others:
http://www.911secrets.com/FAQ.html
Also a small book but nice collection of design photos culled from other books:
Ulrich von Mende, "The Porsche 911 by Fedinand Porsche"
Recently released:
Bongers, Marc; "Porsche data book" Production figures and spec data for EVERY model including Ruf
Paul Frere, Sports car and performance driving