There is a City and Guilds 2365 level 3 text book, but I imagine you'll be provided that when you start the course. Other than that, a copy of the Regs and an onsite guide are really useful to have.
I also have the NAPIT Codebreakers, Guide to the Building Regulations and Guidance Note 3.
There are a fair amount of formula that are worth knowing which should be in the text book.
Swindon Massive is also a good place to look