I mean, I'm not sure you actually need a book to understand it, since it's generally simple enough that it can be understood in one post. Basically, Keynes saw problems arising from underconsumption, the idea that demand falls relative to supply so there is a crisis in terms of realizing profits. This causes wages to fall/unemployment, which in turn lowers effective demand even more. True, if you just sit around long enough then prices will fall to the point where you will start to recover, but that's the entire point of "In the long run, we're all dead". Keynes thought therefore that in times of crisis the government should engage in deficit spending - even if it was totally unproductive, since that at least would get money circulating again. In times of economic growth, the government should cut back and focus on reducing debt. That's it in the simplest possible form.
It should also be stressed that while Keynesianism overlaps a lot with Social Democracy, the two are quite distinct, and Keynes was not a socialist. That being said, Keynes is obviously quite critical of Say's Law since he believed that crises would exist even in a pure market, and thought welfare programs were good for stimulating demand.
Finally, if you want a book, I've heard this is quite good: http://www.amazon.com/Keynes-Return-Master-Robert-Skidelsky-ebook/dp/B0045EPJ76/ref=pd_sim_kstore_5?ie=UTF8&refRID=0FTA68DMFW4SR3XNW8B2