We would encourage you not only to read about mathematics over the summer but also to do some mathematics. Feel free to do any of the practice problems online that seem helpful - these make sure that you are up to speed with all the concepts you need to begin the course. Your college tutors may set you these sheets to do over the summer, but they are good to review even if your tutors don't require it. Additionally, if you've only done A-level Mathematics (or have not studied in the A-level system) you may want to look at the bridging the gap material.
Have a look at the "How do Undergraduates do Mathematics?" booklet which gives you an idea of tutorials, how university-level mathematical study differs from school study, and how to approach problem sheets.
Several of the books in the departmental prospectus are good for bridging the transition from school to university, we particularly recommend:
Lara Alcock - How to Study for a Mathematics Degree
Kevin Houston - How to Think Like a Mathematician: A Companion to Undergraduate Mathematics
If you've done the online practice problems you might also want to look at NRICH's preparation for pure maths and preparation for applied maths. Underground Mathematics has a lot of problems encouraging you to explore various concepts more deeply. If you didn't take STEP you might also want to read through (and do the problems in) Advanced Problems in Core Mathematics: Preparing for University by Stephen Siklos which is available as a free pdf.
You are very much encouraged to view the following video which provides guidance on how to make the most of tutorials.