It is a good idea, but not at $100. The factory service manual covers the mechanical side of the vehicle, and the Fisher Body Manual covers the body itself, of the parts that were originally manufactured by GM. The body parts made by GM consist of everything forward of the windshield, the dash, and the seat frame. Other than that, the rest of the car is all Superior built. I have a lot of FSM's, and I never pay more than $25 for an original. I suggest searching eBay and also the swap meets, I doubt that there is going to be an immediate need for the information, since the car you purchased is in good mechanical condition.
The other books that I suggest that you look for is the illustrated parts manual and the companion parts number book. These are helpful when you are in need of a mechanical part, and want to find a factory original replacement. If you know the part number, it is a lot easier to find what you are looking for. These 2 books are on the expensive side, but worth it when you need that one elusive part. I had one spare, but sold it last winter to another PCS Member. Until you find one, I can always look up the part number(s) for you if the need arises.
Unfortunately, the Superior Parts & Service Manual doesn't have part numbers in it that are of much use, since they used their own part numbers for parts that they purchased from other sources. I never could figure out why they refer to that book as a Service Manual, since it doesn't cover any service proceedures, only parts.