I have two old jet skis, one of which isn't working too well anymore. I'd like to learn how to fix it. I'd probably not be able to sell it for very much anyways. Are there any good books or resources on jet ski repair? I know this is not usually available at your typical community college.
In general, I would dissuade you from just buying 'a book on how to fix jetskis' and reading it cover to cover. This would not only take a lot of your time, but also not give you the specific information you're in need of.
My first check would be for a Clymer/Haynes/Chilton's repair manual for your specific year and model of jetski. There are some available on this site, and of course eBay and Amazon. These would give you much more detailed answers that apply to your specific brand and model, and not generic one-size fits all pseudo answers (4 stroke engines look and work very different than 2 stroke ones, for example).
If you can't find this, I would start googling for specific problems. Lack of power? Poor seals? Bent/damaged impellers? Hull repairs? What exactly are you trying to fix? Once you pick one of these areas to address, start searching for replacement parts or guides to address each area.