1. The Book of Five Rings (Go Rin No Sho) was written in 1645 in South Western Japan by Miyatomo Musashi. I have a 2003 edition translated by Ashikaga Yoshiharu and Rosemary Brant, published by Astrolog publishing house.
2. There have been many extraordinary claims made for the book, beginning with the author's claims in the text, and continuing throughout it's history, culminating with the cover blurb saying that the book is "the cornerstone of Japanese culture". However what we should always bear in mind is that the book is essentially a swordfighting manual.
3. It is a tremendously influential swordfighting manual. Musashi had quite a reputation as a samurai, which means that, amongst other things, he had killed lots of people. His book makes clear that his system is a philosophy and a way of life, although the aim, is always is to cut your enemy. It has a reputation as a very good swordfighting manual.
4. A swordfighting manual is only useful if you put it into practice. Musashi is aware of this. He peppers the text with instructions to study, or practice or understand things. He often says that it is difficult to explain what he means, and understanding must come from using what he says. This has given the book a reputation for esoteric, or hidden wisdom. I don't know for sure, but I have a strong suspicion that much becomes clear if you pick up a sword and practice with it.
5. So what do we have? Some discussion of weapons, uses, stance and movement. Suggestions that there is no one good weapon, stance etc. as all depends on circumstances. Some criticisms of other schools of swordfighting. Some talk about using spirit and mind and practice and professionalism to cut men down. And hidden amongst it some interesting ideas about how all this fits together into a way of life, which is then cleverly undercut in the final, shortest book, the Book of Void.
6. The book, it seems, was based on Musashi's own teachings to his students. It seems that he intended to pass this only to his successors, but somewhere along the way copies were made and it became widely distributed.
7. Despite it's ambiguous and esoteric language, the book is actually pretty straightforward, no-nonsense and short. I can't say how useful it is as a swordfighting manual, but as an insight into samurai thought and practice it is very interesting.
Read This: If you are interested in swordfighting, samurai or Japan, or just want a short flavourful insight into the mind of a man in a now gone culture.
Don't Read This: If you get frustrated by by instructions that you can't follow, descriptions that make no sense or have no interest in Japan etc.
Also: A free online translation can be found here.
 As someone who does not use a sword any more and was never more than a novice I am unwilling to state how useful it is for reasons explained in paragraph 4.