'Aircraft Gas Turbine Powerplants' by Jeppesen - one of the best covering the theory aspects and also one of the main reference books for the NZ BGT. 'The Jet Engine' by Rolls Royce is also a colourful book with lots of illustrations and explains some things from a different angle but is not as in-depth. Those two books and a copy of the list of topics covered in the exam would be an excellent place to start.