Martial Arts novels cover a wide range of books, many of them truly bad.

Off hand among the better ones, in no particular order:

1. Piers Anthony's early series on Jason Stryker whose martial art to take on everyone (Sumo, ninja, karate-ka) was old style Judo. It was co written with a Cuban judo-ka.
The odds are you cannot find this, as it was in the early 70's they were published.

2. The Man who Never Missed, Matadora, the 97 steps and all the rest. A truly inspired series with a great martial art creation 'Sumito'.

3. Stephen Barnes "Fire Dance" and several others in that line. Sort of Pro Wrestlers on Super Steriods and intense very ancient martial arts combined.

4. The Last of the Zinja (very good midveal Japanase martial fiction, sort of like Shogun but earlier). Incredibly a good fiction tale.

5. The 145 books of the Destroy Series (Murphy and Sapir) and the fan fiction site http://www.plansofmice.co.uk/destroyer/index.html

Of course I've contributed fan fiction as I enjoy their martial arts take.

6. Musashi - before the 3 movies was the book, and a good read. It is a soap opera too, but gets into the Japanese sword mythology of Mushashi.

It will take work to find them, but they are fun and interesting martial arts tie ins.

Check out any number of the books by Eric Von Lustbader.

Black Heart
Jian
Etc.

I second Victors suggestion of reading Musashi (the novel, not the 5 rings... which is good as well but a hard read)

-Jeff