'The Atonement' is small, accessible, logical, and informative.
None of those descriptors is necessarily bad or good.
Bad things can be logically written (Hitler followed the rules of logic, y'know); good things come in small packages sometimes; bad things are sometimes on the bottom shelf (the whole point of propaganda is to make misinformation accessible); and some information can be poorly packaged (think 'boring textbooks').
These words are just the facts (at least, from my impression of the book). I'll elaborate on whether or not these things were good or bad:
Small
It's important to note that this entry in the 'Cambridge Elements in the Philosophy of Religion' series is not a comprehensive report on all of Dr. Craig's research, knowledge, or personal insights on the subject of the atonement. Page-number constraints in the series only allowed for the most concise and pertinent information in the space provided. While I was left wanting more information and explanation about some topics, this book can hardly be faulted for such a thing. It's packed with content, but not in such a way where it feels overly dense. Which leads into the next descriptor...
Accessible
Dr. Craig has a process with all of his subjects of study wherein he researches a topic for years (sometimes more than a decade) and then begins methodically compiling and distilling all that information into forms that other people can consume. He usually writes multiple books on each subject: some for the purely academic crowd, and, fortunately, others at the popular level. 'The Atonement', in spite of 'Cambridge' being on the cover, is thankfully one of the popular-level books (with more academic works on the subject forthcoming). Anyone with passing knowledge of basic Christian doctrines can comprehend the information in this book. Dr. Craig is lenient with his technical vocabulary (that is, he tones it down in these popular-level books), and takes the time to clearly define things in simple terms and provide examples. If you are the kind of person that knows what the atonement is in Christianity, or that such a thing is even a... thing, then you already have the necessary baseline of understanding to read this book.
Logical
In terms of structure, I don't think it will ever be possible to fault Dr. Craig in any of his writings. His approach to laying out his subjects of research is probably one of the most methodical and common-sense of any professional philosopher - which is strange to me; some philosophers, whose literal job it is to make sense of the metaphysical world around us, simply cannot write logically in the least where a layperson could follow their thoughts. Not to stray too far off-topic, we can be thankful Dr. Craig is not one of those philosophers. In almost everything he does (books, debates, articles, podcasts, etc.) he lays out terms, gives their definitions, combs through the relative historical thoughts on the subject, and then synthesizes all of these things into either his own personal conclusions, or the possible conclusions that would be rational to infer from the information. In this case, being exclusively a Christian-based subject, Dr. Craig defines 'atonement', examines the biblical data concerning it, takes us on a time-journey through the different approaches to the atonement through history, and then assesses the information and defends what he sees as the correct view (or the most logical view that presents itself). If anyone gets lost in this book, you can't blame the structure or layout of the arguments, at least.
Informative
Finally, 'The Atonement', in spite of its size, packs a large punch. Dr. Craig has never been one for 'entertainment' (unless you consider watching atheists consistent being defeated in debates entertainment), and so this book will likely not hold the attention of one looking for such a thing. However, the point of the book is to inform, and it does that. I came away from these ~95 pages knowing more about what I don't know about the atonement than what I did know before I started reading it - in case I lost you there: this is a good thing. Dr. Craig does indeed provide a tremendous defense of the Penal Substitutionary Theory of the atonement, but, more importantly, he challenges the lay-reader to go deeper with their thoughts about this doctrine and Christian doctrine in general: there is always more to learn, always another perspective to consider, always another counter-argument to defend against, and always another aspect to the Faith to dig deeper into. You won't find many jokes or anecdotes in this book(let), but you will find challenging philosophical and theological ideas to make you reconsider (or consider for the first time) what you thought you knew about the atonement - and, be honest, what else were you expecting from a book like this?
Conclusion
This book is receiving five stars because:
1. It accomplishes what it sets out to accomplish without any missteps.
2. It packs a lot of solid information in while still keeping its primary, non-academic audience in the game.
3. It is an indispensable resource for those seriously interested in the essentials of Christian doctrine but are maybe only beginning - I took notes on it and will be referring to it and them numerous times in the future, guaranteed.
4. Dr. Craig has provided what virtually no other Christian philosopher has done in... I guess ever: a solid, biblically-evidenced defense of the doctrine of Penal Substitution - and it's fantastic.