The original, single-volume guide by Thomas Tomosey is a good place to start. Later revised into a 2-volume set, that may also be worthy, but I haven't had the chance to examine it. Tomosey assumes no prior experience, and offers many tips on improvising one's own tools. Book contains both fundamental repair techniques as well as more camera-specific tips.
Camera Maintenance & Repair: Thomas Tomosy, Michael McBroom: 9780936262093: Amazon.com: Books
Let's Try! Repair and Maintenance of cameras by Michio Ozeki / Japan Hobby Tool, available in both English and Japanese. Still in print, so if you want a physical book, you don't have to pay a collector's premium for it.
Let's Try! - Repair and Maintenance of Cameras
Both Tomosey and Ozeki concentrate on older manual-focus cameras. While newer cameras can generally be serviced too, a broader range of skills and tools may be needed, and newer components and materials such as LCDs and 1980s synthetic materials don't always age well. Pentax ZX/MZ-series cameras would be a poster child for the latter, but online DIY guides for these cameras exist, and I've used them successfully.
Am also aware of Japanese-language guides for fixing "junk" cameras which can sometimes be had cheaply, but I have not examined these.
For your first repair attempts, I'd strongly suggest starting with a cheap camera that you're not afraid to damage at least a little bit. Because until your skill improves, you may find that your tools tend to slip and gouge stuff a lot. So set aside that beautiful but gummy old camera for later when you are more confident, and concentrate first on salvaging junk cameras.
