This title is the part of the popular "Test Builder Series" designed to help improve student's exam performance and increase language competence. Task types familiarise students with the tasks they will face in the exam, further practice and guidance pages build confidence in answering them and an expanded answer key gives clear explanations as to why the given answer is correct.
This book basically consists of prior Cambridge Proficiency exams and their correct answers, though it's also explained why one answer is correct and not any of the others and they give advice on how to perform at the exam as well, which can be really useful.
Without a shred of doubt, this book comes in handy if you're preparing for the said exam, although I would personally recommend complementing your preparation with other books, such as Proficiency Objective Student's book.