Hi

There is no official study material. However I believe that knowledge of Windows, Linux and web apps is required. So if you were to read Hacking exposed Windows and Linux Editions, network security assessment and Web application hackers handbook, these should suffice I believe

 

There is no material for CREST I would say go to the site and download the course details and work through this. There are some courses too that can try help you with the CREST exam like one run by 7safe but are expensive.

Alternative there is also the Tiger scheme that is very similar to CREST but there are course and they are better value for money. (IMO)

Are you looking at taking the CRT or full CREST exam ? Also why are you taking it ? do you work for a company and they need it or you doing it off your own back ?