This book is too thin to be a reliable guide; and frankly, if you want to learn the law, why would you rely on a journalist? (which is what Quinn is).
Quite simply, the law of contract is a complex subject, and you just cannot make it all that simple without misleading the reader. Moreover, there's nothing in this book to help you when you want to research an essay - it's just Janet & John stuff, with no references to scholarly writings that you can read to help you understand the subject better (and get better marks for referring to). If you want an introduction to the subject, then try McKendrick, then for more detail you can go on to Cheshire, Fifoot & Furmston or Treitel.
This book might be all right at A Level, but if you have got as far as university and you still need a book like this, then you are never going to cut it as a lawyer, so you should cut your losses now.