You might want to check out "Understanding Flash Photography: How to Shoot Great Photographs Using Electronic Flash" by Bryan Peterson.
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I'm thinking you have not read it? At least, I do have to differ strongly with that recommendation. Peterson's Understanding Exposure was good for beginners, every beginner should start there (no flash in it however). This Understanding Flash book does have a few good things in it, but it is about using the Nikon GN mode found on a few flashes (SB-700, SB-800, SB-900). Flashes without GN mode are left in the dark.
And that is of course direct flash only. Maybe nothing wrong with that, but it is an extremely limited view of flash.
We can always compute Guide Number ourself, but this book makes absolutely zero mention of Guide Number, or what it is, or how to use it. It only uses the GN calculator mode found on a few flash models. You can see
Understanding Guide Numbers, including GN Calculator about Guide Number.
Even worse, this book ignores all TTL, which is not an appropriate approach for beginners. Speaking of reflective metering, it simply dismisses TTL, saying it might not always be accurate, without any mention of how to correct it (flash compensation). The same is of course true anytime of our reflective camera meters.
Yet, amazingly, for bounce flash, it just says "open up two more stops" over the direct flash value. Like that is going to be accurate. LOL
TTL and/or bounce flash are tremendously important. So is Guide Number if you're going to use it.
In my opinion, this book is a much better try:
David Busch's Flash Photography Compact Field Guide (David Busch's Digital Photography Guides): David D. Busch, Ed Verosky: 9781133597162: Amazon.com: Books
It actually covers stuff, but it does not mention Guide Number.