This book actually has two translations by Thomas Cleary of two books from Japanese martial artists. My thoughts on both and a short comparison are below.
The Book of Five Rings is a pretty good insight into a disciplined mind and professional samurai from 17th century Japan. A lot of it is practical advice and there is some spiritual Zen leaning in there too but I would not go as far to say it is required leadership reading material in the same way as The Art of War by Sun Tzu but no martial artist should be without this book.
The second translation in the book is The Book of Family Traditions on the Art of War by Yahyu Munenori is far more flowery and makes more sense if you have an understanding of buddhism otherwise the section on existance and non-existance may (or may not be ;-)) be tricky to grasp.
In comparision the first book is plainly superior to the second in the manner in which it is written and executed. It's plain talkng and easy to grasp with none of the flowery language prevalent in the second.