Wayne Mahaffey has a lot of the manuals reproduced. As for originals, they pop up on Ebay and craigslist from time to time, but get pricey. As far as history, there is a gentleman named Pat Williams that used to run a website with a lot of info, but this site is no longer around. If you use "thewaybackmachine", you can find some of the info archived.
As for books, the only one I am aware of is "Simplex Servi-Cycle, America's premier light weight motorcycle" by Gary Wollard. Gary passed away in early 2011, and the book is no longer in print. His son Adam may have some left for sale; search this site for contact info on him.
Rick