Focus on getting a basic understanding on how things function first. Than once you understand the mechanisms you start to understand the how part of the question. After this you can start reviewing relevant research and learn how to analyze the data and draw conclusions. This is your best tool you can have. An actual understanding. Then as rodja has pointed out there are some amazing coaches that have years and years of experience but as I said earlier read everything! Don't limit yourself. Even if it is something you will disagree with read it still. Everything helps to broaden your horizon and understanding and will make you a better coach. You will never stop learning in this field.
If you want a textbook like book I would go with supertraining over the NSCA book
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