As mentioned above, the pinrepair.com site is an excellent resource. Take it from someone who is working on their first pin (well, my first pin in 20 years). There is a lot of information to digest there. I printed all 300+ pages of information for my game and have it sitting right next to the game manual.
Also, if you have a specific question for a specific game, check the the rec.games.pinball newsgroup. Search first...and if there is no answer, post your question.