Bluetooth LE development is getting very popular with an increasing number of learning resources. Here are some that have appeared recently:
Erik Hellman, a developer, has two useful articles Bluetooth LE for modern Android Development part1 and part2. He covers Android versions, Bluetooth GATT, PHY, Android Device Manager, pairing and bonding.
Litum have a new article What is Bluetooth Low Energy (BLE)? How does BLE work? that’s higher level and less technical covering Bluetooth LE device discovery, differences to Classic Bluetooth, how positioning works, Bluetooth range , usecases and industries.
Zephyr, the embedded systems OS have also recently updated their Bluetooth documentation.