Books, undoubtedly.

I find the Napoleonic/Revolutionary wars fascinating and as such attempt to gather a wide range of books on many disparate topics regarding it.

I find there are two main types of books, ones written much more recently and are summaries on topics and others which are more primary sources. For summary books the level of detail and specifics varies, from books on Napoleons invasion of Russia to the organisation, equipment and actions of the Russian light infantry during the 1812-13 campaigns.

These books draw on many sources primary and otherwise to collect together information regarding one particular event or thing that would otherwise be scattered across dozens of other resources that are not dedicated to that topic but otherwise mention it or primary sources that are often memoirs or the like and so are filled with information that is not immediately relevant.

Then there are the many more primary sources, these are are things like the memoirs of soldiers who were there. Examples include 'A Dorset Rifleman', a collection of the Memoirs of Benjamin Harris and 'With Napoleons Guard in Russia, the Memoirs of Major Vionnet 1812'. Care must be taken with these to be aware of possible discrepancies however, as they may be translated or assembled from many notes and writings that may be subject to interpretation or bias from the Editor that probably isn't intentional.

Then there are the actual books from the period. These are obviously hard to come by but thanks to resources such as Google Books many are now readable online. This allows for a small view into the time through the eyes of the writer. Great examples of which are some of the Original British Drill manuals. (I link them here on a post in /r/UKhistory )

Many books are available second hand for inexpensive prices, some however are no longer in print and conversely are available second hand but at extortionate prices. That doesn't stop them from turning up in second hand bookshops for only a few quid from
time to time though, I always check the military history section of any second hand bookshop I come across.

It is possible to use the internet to research some things but many websites lack adequate detail or sources and consequently books are the best option in my opinion. Better still, books list their sources and this can lead you on to other books on the same period.