Keynesian economics is the basis for much of the current economic philosophy. When people argue about how to get out of a recession or how to increase jobs, they are arguing over Keynesian economics.
Despite the importance of Keynesian economics, many people are neither aware of the source nor familiar with the underlying theory. Much to my delight, John Keynes' book The General Theory of Employment, Interest and Money is available in the public domain. Some of the reproductions aren't of the highest quality, while others are better.
I have used the overall ideas from this book on many occasions as a starting point for many of my economic ideas without ever having read the book. I have settled for secondhand information about the theories and been forced to 'reinvent the wheel' when there were portions I didn't know. Now that I have some time, I plan on going over the book in minute detail and discovering not only what they theories predict, but more importantly what they are in response to and why they are expected to work.
If you are interested in following along, be warned that it will be slow going. Many of the ideas take time to understand completely. Other ideas are rooted in long established economic theory and jargon that needs to be understood before proceeding. Often times it is necessary to take a break from the book; define words, concepts, and ideas; only then returning to the book to understand the theory in question.
The task isn't for the faint of heart; but if you are interested, I am using a good copy of the book I found on Amazon.com. While this book is in the public domain and available for free at Project Gutenberg, the formatting of the Amazon.com copy seems to be really good making the study easier.
If you do decide to spend money on a digital copy of this book, beware: since this book is available in the public domain, there are some people that are taking a poor copy from the internet, putting it into e-book format, and selling it compete with all the errors found on many of the available public domain copies. I have linked to a copy of the e-book that I am using, and it seems to be an excellent copy complete with footnotes, hyperlinks, proper formatting, and correct spelling and punctuation.