The Wikipedia.

https://www.icrc.org/en/who-we-are/history

When I posted this in December 2016 there were over 5,300,000 entries just in the English language Wikipedia. https://en.wikipedia.org/wiki/Main_Page

Though, of course, the quality of entries, varies, and one has to read everything critically, many Wikipedia articles are the best encyclopedia entries on the subjects concerned, and, of course, they are free to all.