If you have a local Mustang club, that would be a good place to get comfortable with whatever level of maintenance you can handle yourself. The Mustang is a desirable collector and should be treasured and respected. Next do a lot of research, ie. Google and as was mentioned, the library. Next research how you can apply synthetics to the engine and transmission once everything is running in good condition, synthetics will save you time and money in the long run.

I use a Chilton Manual on my car, but 200,000 is high,

there's a term you should know called 'Totaled' when repairs start adding bills non-stop, its cheaper to buy something else. Look at the price of the car currently, and my general idea is 2x to 3x the repair is the value, consider reselling.

Chiltons Repair Manual.

You check with your local library. They have service manual available. See what you like then buy the book for keeps/reference.

I have a v6 mustang with 200k miles and repairs are getting costly. I also think it would be good to know these things. What books do you recommend? What are some other realistic options?