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:

https://www.amazon.com/Bluetooth-Low-Energy-Developers-Handb...

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... http://www.amazon.com/Bluetooth-Low-Energy-Developers-ebook/...