The Fighting Man of Japan: The Training and Exercises of the Samurai by F. J. Norman
My father passed this one along to me. It is a very short work, published in 1905 by an English military officer who was sent to teach the Japanese about modern warfare. Along the way, he was exposed to, and studied, several Japanese martial arts, most notably some form of Kenjutsu (he consistently refers to it as fencing).
If I had a complaint about this book, it is only that its very short. Norman really just gives a cursory overview of what I'm sure was a more in-depth understanding of the Japanese martial culture and tradition at the time. It's clear that he has a great deal of respect for the Japanese warrior traditions, though he avoids succumbing to the modern assumption that they are somehow innately superior to European methods. His discussion on that particular subject generally boils down to "it depends".
One thing that I found interesting was Norman emphasizes on several occasions that the Japanese systems, in contrast to the European methods of the time, place LESS emphasis on form and precision, and a greater emphasis on effectiveness and free play. I'll have to dig out actual quotes when I get home, but its a striking reversal of the stereotypes of both methodologies, from a guy who was actually there.
This is a good, quick, read for anyone interested in Japanese or European military history or martial arts. Check it out.