Hello James,
While I agree that one can make great music without reading it, two thoughts (three, actually) :
1. From what I understand of your immediate needs (improve your knowledge of basic percussion, interested in the military / marching rythms), I would say that, indeed, knowing how to read music would have help you to get there faster : all you'd have to do is go to the nearest score shop and just take a look at how it's done. Military / marching rythms scores are for the most part basic stuff. They also are pretty common and affordable.
Think of it this way : reading english allows you to look for a cooking recipe on the web. After that, you can adjust to your taste, improve your own recipe etc. Who knows, maybe after a while you will feel like taking some cooking classes to go further ! (that would be the harmonie / contrepoint etc.)
2. Reading music, and learning how to do so is easy. Children can do it ! That means that not only can you do it, but you can do it fast. It is a fairly basic, logical and effective language. Once you understand the principles, your skills will grow with your needs as you naturally put more effort into it.
Why not begin with rythm if you wish ! It will be instant gratification. I garantee you that with as little time as 15 minutes a day it won't be long before you're able to look at a non-melodic percussive score and understand what's going on, thus being capable to reproduce it or adapt it to your taste.
3. Meanwhile, going to (decent !) live concerts will help a lot. Watching at the percussionists at play while you're listening will help you distinguish the differents instruments better, along with the way they interact together. It's easier on live than on record when you're not familiar with it. Try and go to some (real !) musical school, see if by any chance the percussions teacher would let you assist at some classes. Watch the student work. Meet them. Ask questions. You will definitly learn a great deal of things. Also, I would consider rehearsals a place to be. Loads of things to learn there too.
Hope this helps [:)]
Regards,
Alexis