The reading list is also an excellent resource but not entirely free.
I don't know of any modern books or such that are free to read however I know many books can be purchased second hand at drastically reduced cost. In many cases the postage being the most expensive part.
Still, I can recommend a few of the original drill manuals and accounts for the British Army. These have been written so long ago that they have entered the public domain and are free to access.
Random Shots of a Rifleman - John Kincaid
A Treatise on British drill and Exercise of the Company - Antonio Suasso
The Discipline of the Light Horse - Robert Hinde
Instructions for the drill, and the methods of performing the eighteen manoeuvres - John Russel
A practical guide for the light infantry officer - Thomas Cooper
A manual for volunteer corps of cavalry
The Little Bombardier, and Pocket Gunner - Ralph Willett Adye
I know these may not be exactly what you are looking for but I recommend having a quick explore of google books as I have found the service to be superb.
I have some recommendations for books that you should be able to pick up relatively cheaply on Amazon.
Weapons and Equipment of the Napoleonic Wars - Philip J Haythornthwaite
The Napoleonic Source Book - Philip J Haythornthwaite
Many of the Osprey military series such as
Wellington's Light Cavalry - Bryan Fosten
and
British Napoleonic Artillery - Philip J Haythornthwaite
Although I focus mainly on the British military of the period I would be happy to reccomend more specific books if you are interested.
Edit: Fixed a link
There are two very good websites that have a number of articles, book reviews, and forums that give a good insight into the Napoleonic period.
The International Napoleonic Society's website is a bit tricky to navigate, but there is some good stuff there.
The Napoleon Series is one of the oldest Napoleonic websites and likewise has a large number of articles, essays, and its forum does have lively discussions between a number of authors on the periods (Kevin Kiley, Digby Smith, Rory Muir, etc.).
Both websites are free and well worth studying to learn more about this particular period.