Let's put it this way:
I have been working on cars since the late 1960's (yeah, I am an fossil). I have NEVER owned a car that I planned to keep more than six months that I did NOT own the FSM for it.
The FSM is SO VALUABLE as to be almost indespensible. Even if you don't do all your own work, there are TONS of things you can learn about the design and operation of the vehicle by thumbing through the pages of the FSM.
I would wholeheartedly recommend that, if you have the money to invest in lifts, skidplates, bigger wheels/tires, etc., the FIRST THING you buy is the FSM. It's a God-send when your XJ starts acting up, and you are trying to figure out it's mysteries.
I bought a Haynes manual when I bought my '92. While it's "OK", it just does not have the detail I was looking for. So, I made the plunge after a couple of months and spent the $60 on the FSM. Since I plan to drive the wheels off my XJ, it's the best money I ever spent on the Jeep, without a doubt.