one can easily clear the CCNA Security (210–260) by using the promising study materials or books. There are many books available, but it is always recommended to start with the official cert guide. Here is the list of books which are good to start
1.CCNA Security 210–260 Official Cert Guide 1st Edition by Omar Santos, John Stuppi
2.CCNA Security (210–260) Portable Command Guide (2nd Edition) 2nd Edition by Bob Vachon
3.CCNA Security Self-Practice Review Questions for the security track: 2015 Edition by ExamREVIEW
Check out the more recommended book for CCNA security. (http://www.simulationexams.com/books/cisco/ccnasecurity_books.htm)