I have read quite a few books on the Napoleonic wars but one of the best reads is "The Battle" by alessandro Barbero. This is specifically about the battle of Waterloo and really is a fantastic book that really depicts the battle in gritty realistic terms. The book is extremely well written and gives a real sense of what it would have been like to fight in that period. lots of contemporary accounts and anecdotes and very well written. The writer is Italian so there is not the national bias that you get in some accounts. Highly recommended if you have any interest in the period at all !!
War and Peace by Leo Tolstoy is wonderful if you have time.So is Who Won Waterloo by Barry Von Danzig.For the period at large there is a wonderful book called The Struggle for Power in Europe (can't remember the author) which goes into a lot of detail about the economic and social effects of the wars.These are all pretty heavy reading but definitely worthwhile.Clausewitz, On War is another wonderful book, although it is not historical it does put you in the mindset of the times around and after the Napoleonic wars. It is open source material on the net too.Hope that helps.
"War is the continuation of policy by other means." Carl Von Clausewitz
To be exceptionally low brow and open to ridicule I'm going to say I learned a lot from the factual parts of the Sharpe Companion and the much later Sharpe Companion Vol. 1. A lot is good first hand accounts of life as an actual combatant in the British army at the time which you don't really get in most books.
From the British side i'd read Mark Urban: Rifles and Richard Holmes: Redcoat. Both are very good scholarly but accessible works about the british army written mostly from contemporary sources and letters. Both authors have done a great job to weave together a story sometimes fixing on the experience of one man or regiment but then relating it to the experience of the armies as a whole. I'm not sure what i'd recommend as an 'overview book' but both of these are excellent 'bottom up' books! Not sure about other armies though!Hope that helps!
I'm just reading the aforementioned Redcoat by Richard Holmes and very much enjoying it. He makes a point about not being obsessive over period detail and is more interested in the soldiers themselves, which I think is good. I always try to get to know a bit about the men inside the uniform to get a real feeling for a period.