Recently a colleague wanted to know from where should he start reading about LTE/SAE security. The obvious answer was 3GPP TS 33.401 which is the specification and provides complete details. It seems that some people get scared when they start looking at the specs and in that case it is preferable to have a book chapter or something similar that could provide useful information.
Agilent, the T&M manufacturers released a book last year on LTE and the chapter on the Security is freely available on the web which I have also stored on the 3G4G website. It is a good starting point and provides basic details that technically minded people may find useful.
You can have a look at the Security chapter here.