I used - CCNA Security Official Exam Certification Guide - Michael Watkins & Kevin Wallace.

But other i know have used these also.

Implementing Cisco IOS Network Security - Catherine Paquet
CCNA Security Study Guide - Tim Boyles

I did my CCNA SEC right off the back of the old CCNP ISCW exam - which was pretty much the same exam and i was working with ASA most of the day at the time.