Haynes manual is pretty good if you are thinking about buying a car and want to know how hard it is to work on. I keep one in the TranSport in case I need something quick on a trip since it fits anyware.
Also have the full FSM and a copy of the Parts and Accessories catalog and a copy of all the TSBs (technical service bulletins) issued after the FSM was published for use when I want to do something major. If you are serious about keeping and working on an older car, you really need all three.
Service manuals are usually relatively easy to come by (usually even available new from Helm for a stiff price). P&A and TSBs are a little harder to come by. Hardest are the Delco Radio service manuals for anything newer than 1985. Impossible is anything on the ECM, that requires a good hacker (GM-ECM is a good place to start).
There is an increasing movement to get the FSM and P&As online - the Reatta club (www.reatta.net
) has the 93 P&A, assorted TSBs, and FSMs for three of the four years the car was built all with the knowledge and permission of GM - tremendous amount of information on the 3800 "C" and "L" drivetrains including 440T4, 4T60, and 4T60E transmissions.