IMHO, these 2 links are what I consider to be main roots of the modern agile movement:
If you are looking further, this paper looks pretty exhaustive with more than 50 year history of agile:
The paper refers to a very approachable book that compares / contrast different agile methodology. Worth a gander too:
- Larman C. (2004). Agile and Iterative Development: A Manager's Guide. C. Alistair and H. Jim, Pearson Education, Inc.