"The Dream Machine" by by M. Mitchell Waldrop (https://www.goodreads.com/en/book/show/722412.The_Dream_Mach...) is a really good book about J.C.R Licklider, one of the perhaps lesser-known individuals who helped create the modern computer revolution, in no small part as an ARPA (later DARPA) director. By reading the book, I learned that the changing of directors is by design, and an 8 year tenure looks like a problem with finding a suitable candidate to fill the role. 2 years seems more in line with how it was set up. I really recommend the book for anyone interested in the history of computing, the internet, or DARPA.