When was the first Martial arts book written?

When was the first Martial arts book written?
It's difficult to determine since ancient Martial Arts were only taught within small circles, and in secret. There were no books as we know them today, and even the ones that have been translated to this day do not have any definitive date of when they were written.

However, Samurai Miyamoto Musashi's Go Rin No Sho (Book of 5 Rings), written by Musashi around 1645, is considered one of the oldest written works on the subject still around today. It is a must-read for any aspiring Martial Artist.

Though a work of military strategy rather than Martial Arts. Sun Tzu's "The Art of War" which was published in 1621, can also be applied to certain aspects of Martial Arts training, though it's up to the reader to determine how his philosophies can be utilized by the practitioner.
The Bubishi, another treatise on warfare, was published during the Ming dynasty (1366-1644).

The earliest karate book is probably the karate Bubishi which was published during the following dynasty, had parts which can be traced back to several Chinese Generals and were written in 1561. Patrick McCarthy's recent revised translation is probably the best and is readily available. It contains a great deal of practical advise, discussions of pressure points and some kata.

You may also want to consider the 'Kihyo Shinsu', a Chinese martial arts manual written by Chuk Kye-kwang of the Ming Dynasty. This book was obtained by King Sunjo (1567-1608)
Another book to consider is the Korean 'Muye Dobo Tongji' written by Park Je-ga, Duk-moo, and Pak Dong-soo as it is a compilation of martial arts tactics from as far back as 1392. The edition translated by Sang H. Kim is a great version.