I'm hoping to do some relevant reading to boost my personal statement. Are there any books which are considered a 'must read' for studying psychology? If not, does anyone have any suggestions?
There are no set 'must read' books for the course. What you could do is look into any introductory Psychology textbook and have a brief overview of the area that interests you and the associated theorists and mention that briefly. I think what would be more ideal, is to put down relevant work experience. Including such information as well as the reasons why you want to do the course and why you would be an ideal candidate are more important. Don't worry about trying to be 'fancy', be clear and concise and that should be more than enough.
Honestly, read what you find interesting. Look into areas of Psychology that interest you (developmental, criminal etc.) and read what the key theorists in those areas have written. Don't read it just for the sake of your PS, the university will be able to see straight through you.
I don't think there are any "must reads"- its not like politics/philosophy. As people above have said, read what you find interesting! I really liked Robert Winston's books personally, and he comes from a very biological perspective.