I've gotten most of mine off the 'Bay. Right now, all I can see there for you is the Electrical/Vacuum Troubleshooting Manual, which I'd jump on right away. It's usually the hardest piece to find and one of the most useful. Your car will have two books as well: One for mechanical and one for body. I'd set up a "favorite search" on eBay for "1987 Mustang service manual" and wait for them to come up. New Ford manuals are sold by www.helminc.com, but they'll be really spendy if they're still available.
There's no substitute for the factory manuals and I've never regretted spending the bucks to get them. I've got complete sets for the Mustang, the Cobra, my wife's Bronco and my Crown Vic. You can't beat them when you're digging into your car.