The problem here is that while Cleary is probably the best-ever translator of Buddhist literature, he is no Zen master. His translation of the actual text of the Mumonkan is supurb. Unfortunately, he gives lengthy commentaries on the cases. He's clearly not a Zen Master, nor a koan student, nor even a Zen Buddhist. He's a well-read translator who hasn't engaged in koan study.
As someone who is personally struggling with Joshu's Mu, Case 1 of the Mumonkan, I can tell you that Cleary is even farther away from it than I am. He's clearly wrong about it. Why buy a book on koans from someone who has never done koan study?
This translation of and commentary on the well-known Wumenguan/Mumonkan is one of Thomas Cleary's finest works. (I also think well of his _Dhammapada_.) As reviewer David Johnston has noted in his excellent and accurate review, it will clear up plenty of the misconceptions about Zen encouraged by people who (deliberately or otherwise) profit from obfuscation. And Cleary's commentary -- based on some thirty years of experience with the koans themselves -- will provide valuable guidance that those professional obfuscators would probably prefer that you not have.
There are plenty of books out there that purport to be about Zen, but as far as I can tell, only a handful of them are genuinely helpful over the long haul -- Reps's _Zen Flesh, Zen Bones_, Kapleau's _Three Pillars_, Suzuki's _Zen Mind, Beginner's Mind_, the other Suzuki's _Introduction to Zen Buddhism_, maybe Alan Watts's _The Way of Zen_ and Stephen Mitchell's _Dropping Ashes on the Buddha_. Cleary's Wumenguan belongs on the shelf next to these.
Cleary insists (correctly) that Zen is not anti-intellectual or anti-reason ("not blind to causality"), and it doesn't encourage the practitioner to dissolve one's mind (or the world) into undifferentiated mush. On that basis alone, quite a few of the trendy "Zen" books currently in print can be tossed directly into the trash.
One fact of which the reader/buyer should be aware: this is the very same book that was previously published as _No Barrier_ (which the back cover of this volume incorrectly calls _No Boundary_). I've had the earlier book since it was first published and I'm glad I didn't buy this one.