The Haynes manual is the first I bought for my 944, way back in 1986. It brought me no insight as to what makes this vehicle unique, and what special tools would allow me to work on the car myself. After I suffered a timing belt failure (all too common to this series), I discovered that there was not nearly enough information for me to fix this problem myself.
I purchased the factory service manual (for $250 at the time) and was able to fix my problems, and to prevent many other problems. There is no substitute for the factory service manual, and it is now generally available on CDrom.
If you have a 944 turbo or 944S or 944s2, this book is absolutely worthless to you, as it has no wiring diagrams or real information that is covered in the factory manual.