In addition to SkySmith's book, I also found a few other information sources to be especially useful.
First and foremost is the Cessna Pilot's Association Systems and Procedures course. I attended this intensive course several years ago (I believe it was either 3 or 4 days). You pick up a wealth of technical information on the skymasters, get some real hands-on experience at looking at the health of various systems, get a nice bound compilation of material relating to the skymaster, and get to interact with a lot of skymaster owners who can share their own insights. This was the single best resource I have run across, and I would highly recommend it to anyone thinking of buying a Skymaster, as well as existing owners.
The other skymaster web sites and message forums are also helpful. Since they are mentioned elsewhere, I won't go over the list again here.
The FAA's aircraft registry (
www.faa.gov) is also helpful for locating potential suspects or just checking on various registrations.
There is a CD that has all aircraft registration information (including chain of title with original documents) and Forms 337 on it. I don't have this, but have seen it. It is a great resource. I assume it is available from the FAA. You can use it to put together a fairly detailed history of ownership and significant modifications and repairs to any airplane.
You can also use the FAA's A/D database to see what A/D's apply to the model you are looking at.
Teledyne Continental motors site will also let you search for A/D's and SB's on the engines (you will need the engine serial numbers). You must register for the free "aviator services" in order to gain access to this.
When contemplating avionics upgrades, eastern avionics site is nice (
www.avionix.com) thanks to their fairly comprehensive catalog, decent pricing, and ease of use.