You learn a lot from other good people and Aussie Pinball.
Basic knowldege of how a machine works is highly recommended and repair involves a process of elimination similar to car repairs.
A very basic book for starters available on internet is "Pinball Machine Care and Maintenance" by B.B. Kamoroff. More specialised works require specific manufacturer manuals or system reference books.
The web and this forum are valuable for chasing up parts, referene and first hand experience.

+1 to that. Particularly the the knowledge shared amongst the AP forum community. Lots can be picked up reading the responses to others problems and when you have your own, helpful advise is never far away.
 I'd still be trying to get my first machine working if it wasn't for AP and in particular Pinister (Blair) who's helped me out with house calls more times than I can recall.