Found in 2 comments on Hacker News
jbonniwell · 2017-06-02 · Original thread
When I was doing Bluetooth Low Energy development for IoT devices, this book was the goto reference for both the embedded firmware and mobile app folks:

onebot · 2013-10-13 · Original thread
This is the best resource on the subject, written by the designer of the protocol. The author is, surprisingly, a pretty good writer. This not only gives you everything you need to know (without grokking the spec), but also talks a lot about why they made the decisions they did.

Highly recommend...

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