The Bluetooth LE specifications are very technical and not very approachable for those wishing to use, rather than create, devices using Bluetooth. Bluetooth SIG, who manage the Bluetooth specifications, have a new document by Martin Woolley titled The Bluetooth® Low Energy Primer. This is the definitive guide to better understanding Bluetooth LE.
This free document covers the history of Bluetooth LE and explains the various layers in the Bluetooth stack. It describes the protocols and profiles that make up Bluetooth LE. It also provides pointers to study guides, papers, hands-on resources and formal specifications for further reference.