You will find that the course notes will provide the material you need. There are only three UK organisations doing the IR exams only so there is not much choice but all are by all accounts very good. If you really want to read ahead then your only choice is to get hold of the ATPL study guides, such as the Oxford Air Training manuals. They run at around £40 for each subject. The ATPL books have far more than you need if you are just doing the IR so you need to check the learning objectives to see what you need and what you can ignore. The sylabus may change significantly in the next few years as there is talk of the EASA IR having significantly reduced theoretical knowledge requirements.
In terms of the current requirements probably the ATPL books on Air Law and Met are the best places to start as the IR sylabus has a lot of both. Air Law is the most tedious of the subjects in my view; if you can manage that you can manage anything.