I am fortunate to have a nice sized library. However, there are some books I go back to again and again. This is a short list of books you may find useful as well. I really love digital editions as well and I wish these books were all available in a digital format.
The Napoleonic Source Book
by Philip J. Haythornthwaite
I love this book, it is jam packed with lots of very useful information for the military modeller. Sadly it is out of print but I am trying to track down who currently owns the rights to the book to see if it can be republished.
A Military History and Atlas of the Napoleonic Wars by Vincent J. Esposito (Author), John R. Elting (Author)
Again, this book is out of print. Buying it second hand can be an expensive affair. But if you want a good overview with lots of fantastic maps you can't go wrong with this.
NAPOLEONIC UNIFORMS: 2 Volume Boxed Set by John Elting and illustrations by Herbert Knotel
This is a 4 volume set that you can still pick up, 2 volumes form Amazon or similar and the others on the second hand market. There is simply no other sets like these to compare. They are absolutely beautiful, inside and out. The range of units covered is massive.
Soldiers and Uniforms of the Napleonic Wars by F. Hourtouille
I really love this book, interesting illustrations, some odd choices. You can see the full list of units in my review.
Uniforms of the Napoleonic Wars by Digby Smith
Not my favourite book by a nice introduction covering lots of units with lots of plates. See my review here.
The Peninsular War Atlas (General Military) by Nick Lipscombe
Wow. This is a beautiful book, as the title suggests, focused on the Peninsula War.
The Campaigns of Napoleon
The major publishing house when it comes to military history. It is also very useful for military modellers. They also are starting to publish digital editions which I am about to try out. The quality can be a bit mixed, some of the titles need an update to incorporate new data as well as some titles that have poor illustrations. Look at reviews around the web and make your choices wisely.