Tuttle Martial Arts has just published a book by Reynaldo S. Galang, the 'Complete Sinawali', a text on Filipino Double-weapon fighing.
Somewhat pricey for its size ($16.95), it provides a very good reference into Filipion Double-weapon drills.
I have a number of arts on the Filipino Stick Arts and cannot think of one which goes into as much detail on double-stick fighting.
Tuttle's series Complete "Shotokan, Aikido, and other titles" are very good presentations on each topic.
BTW, a real hint here, if you want a personal insight to Okinawan Karate, you must purchase the text on 'Complete Aikido'.
Bushi No Te Isshinryu www.funkydragon.com/bushi