Bluetooth Low Energy (BLE) radio chips are ubiquitous from Raspberry Pi to light bulbs. BLE is an elaborate technology with a comprehensive specification, but the basics are quite accessible.
A progressive and systematic approach will lead you far in mastering this wireless communication technique, which is essential for working in low power scenarios.
In this book, you'll learn how to:

Discover BLE devices in the neighborhood by listening to their advertisements.
Create your own BLE devices advertising data.
Connect to BLE devices such as heart rate monitors and proximity reporters.
Create secure connections to BLE devices with encryption and authentication.
Understand BLE service and profile specifications and implement them.
Reverse engineer a BLE device with a proprietary implementation and control it with your own software.
Make your BLE devices use as little power as possible.

This book shows you the ropes of BLE programming with Python and the Bleak library on a Raspberry Pi or PC, with C++ and NimBLE-Arduino on Espressif's ESP32 development boards, and with C on one of the development boards supported by the Zephyr real-time operating system, such as Nordic Semiconductor's nRF52 boards.
Starting with a very little amount of theory, you'll develop code right from the beginning. After you've completed this book, you'll know enough to create your own BLE applications.