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The Speedliter's Handbook is number one on my recommendation list for those wanting to learn the nuts and bolts of how Speedlites work.
Number one on my recommendation list for those wanting to learn how to make beautiful images using Speedlites is On-camera Flash -- Techniques for Digital Wedding and Portrait Photography by Neil van Niekerk.
+1 on both books
Suggestion: Take the time to read through The Speedliter's Handbook cover to cover, you'll miss a lot of great information if you skip around. It's very well written, easy to understand, and flows nicely throughout. Love it!