This drawing (HQ version) shows a result from our research published today in PLOS ONE. It actually is a simple plot depicting the difference between the emotions of Joy and Sadness in approx. 5 million books published in the 20th century (1900-2000). You may have noticed that the peak for Sadness — and equivalently the minimum level of Joy — occurred during the World War II period.
However, for people who despise simplicity, we have also included some more elaborate (and perhaps more interesting) results in our paper. I'm not going to repeat them here, of course, as it makes no sense — that's the purpose of the publication!
If you feel that reading an academic paper is a painful task (even for this easy-to-read, made-for-humans paper), then you might find the following press releases useful: PLoS ONE, University of Bristol or University of Sheffield, Nature.
Acerbi A., Lampos V., Garnett P., Bentley R.A. (2013) The Expression of Emotions in 20th Century Books. PLOS ONE 8(3): e59030. doi:10.1371/journal.pone.0059030