As stated on the NCCA accreditation page, there are NO REQUIRED study materials for the CRCS™.  

Per NCCA guidelines, as the issuing body for the CRCS™, The Wealth Preservation Institute’s role is to create, administer, grade the exam, and issue certificates to those who pass. The Wealth Preservation Institute does NOT offer official study material.

Optional Study Tools

Advisors can seek out and use any study materials they see fit so they can study for and pass the exam. There are many books in the marketplace (although most lack the needed details to help advisors pass the CRCS™ exam).

Michael Kitces and his partner have an online educational course, which isn’t bad, that advisors can take (although most of the content from their course is covered, and more, in the following book). 

Deconstructing Roth IRA Conversions: Myth vs. Reality

Click here to buy on Amazon (Kindle or in print).