John.
The best resource hands down is the Train Control Systems web site with the installation pictures. A picture is really worth a thousand words. http://tcsdcc.com/ After you do a few decoders you will come to realize that the hardest part is removing the shell.
Pete
I pray every day I break even, Cause I can really use the money!
I started with nothing and still have most of it left!
OJT?
Seriously, I don't know that there's "one best" book or resource, simply because there are so many different decoder/loco combinations, as well as different ways of thinking about how it "should" be done.
I'd start by searching the Web. Pick a loco you want to convert to DCC, and check the decoder manufacturers' Web sites for instructions covering that make/model of loco. Even if you pick, let's say a Brand A decoder, the Brand B Web site may have instructions for installing a similar decoder from their own product line.
And remember, Google is your friend. Again, as an example, I just searched for "dcc decoder install p2k gp30" (without the quotes) and got about 3790 hits. Not all of them will be helpful, or necessarily even relevant, but it doesn't take much effort to skim over them and to see if they merit further investigation
What's the best book or resource to learn how to install decoders in locomotives?
http://www.kalmbachstore.com/modeltrains-railroading-model-railroading-books-wiring-electronics.html
I see those but they all basically look the same.
John F.Perham, MNRailfan Photography | Modeling HO Scale Trains & O Gauge Trains