It depends on if you are just looking at standard maintenance or are restoring or rebuilding part of your Challenger.
DetMatt1 is correct that the FSM is the best staring point. You can't learn too much so read everything that you can. Knowledge is power.
Not sure how detailed the FSMs are but i can't live without wiring diagrams which are available for free in various places online. Also the ATSG torqueflite transmission service book (can be found online). Others i rely on are "how to rebuild small block mopars", the parts interchange book... and the internet.