Factory Service Manual
If you perform anything more than basic maintenance on your vehicles, seriously consider investing in the factory service manuals. The DIY books that are sold at auto parts stores often include several years and models. The information can be too generic, inaccurate, or incomplete. The factory service manual, though expensive, will have all the needed information.
I bought the FSM for my 1984 Chevy and it was a life saver on more then a few occasions.
And yeah, it was about $50 even back then, but saved me hundreds in labor costs.
Definitely nothing beats the FSM, especially for electronic and wiring issues. I got a complete set for my Mazda b4000 (actually Ford Ranger manuals) on eBay for about $80 brand new. Has saved me much more than the price in time and frustration.
Barring that, a free alternative can be found through your public library. Many libraries have subscriptions to online service data sites. There you can find manuals, TSBs, and recalls for virtually any vehicle. I access mine from my home computer. You just need your library card # to log in. Great for helping friends and relatives with vehicle issues.