I just bought Niccolo Machiavelli's "The Art of War" (Da Capo Press) from Borders for just this reason. He talks about the composition and fielding of armies: standing armies vs. mercenaries, how many men in a legion and of which type, where to place cavalry, and some battlefield tactics.
Since this book has long-since passed into the public domain, you can see it at
http://www.constitution.org/mac/artofwar_.htm among other sites.
It's a little difficult to find things since there isn't a table of contents. But fortunately, Amazon.com provides one:
http://www.amazon.com/exec/obidos/tg/de ... i=contents
Now, this was mostly Machiavelli adapting ancient Roman techniques to the middle ages. The real reforms of the medieval army were performed by Maurice of Nassau and Gustav Adolphus, but I couldn't find books on them specifically at Borders.
All this said, a Google search on "medieval army" turned up some interesting results.
De Bellis Antiquitatis (medieval armiy orders of battle):
http://www.fanaticus.org/DBA/armies/medievals.html
This one is awesome for the large variety of ancient and medieval armies, all presented for wargaming.
Internet Medieval Sourcebook:
http://www.fordham.edu/halsall/sbook.html
At first glance I didn't see an order of battle, but it might be in there somewhere!
OSPREY! I love this company. They are a great source of information on armies of all kinds, with many illustrations. You can find these at Borders, gaming stores, and your local library.
For example, Medieval European Armies at
http://www.ospreypublishing.com/title_d ... 457~per=41
Et cetera, et cetera. Google is your friend.