If you really want to get into the details about a specific language, anything from the three "History of Programming Languages"[1] (HOPL) conferences is great. These conferences, convened every ~15 years, have talks by the creators of various seminal programming languages recounting their histories.
You can probably find most of the HOPL papers online as well as recordings of talks from the later conferences. There's a lot of material! Personally, I really liked "A History of Haskell: Being Lazy with Class"[2][3], partly because I like Haskell and partly because Simon Peyton Jones is such an engaging speaker.
In 2001 MIT had the Dynamic Languages Wizards Series [1], which consisted of three panels of luminaries in the field (videos and participants names on the linked page). A lot of history was discussed in their 5+ hours.
They are also on YouTube:
Panel on Runtime: Richard Kelsey, David Moon, Tucker Withington, Kim Barrett, Scott McKay [2]
Panel on Compilation: David Detlefs, Will Clinger, Martin Rinard, and Mat Hostetter [3]
Panel on Language Design: Paul Graham, John Maeda, Jonathan Rees, Guy Steele [4]
These might be obvious already (and hence redundant), but many computer history museums have sections covering software and programming language history.
There are several old primary resources on C's predecessor, B [2]. Structured programming was newish, so it's fun to read these descriptions of newfangled "while" loops.
While not a history book, per se, "Masterminds of Programming: Conversations with the Creators of Major Programming Languages" [1] is a pretty great read. You can get a ton of historical context from reading what the language designers were thinking when they created their languages.
Andreas Stefik gave a talk titled The Programming Wars[1] at my school -- I thought it was a fascinating overview and the video is available at the link. If you already are pretty familiar with the history of programming languages (I'm not), it might be less interesting for you.
Masterminds of Programming is a good read. It contains of interviews with lots of creators of programming languages. The interviews cover a broad range from history to design to philosophy.
Or the mailing list archives for the many OSS languages. The gcc archives are probably chock full of fascinating stuff. Unfortunately this would only take a researcher back to the 90's or maybe somewhere in the 80's.
You can probably find most of the HOPL papers online as well as recordings of talks from the later conferences. There's a lot of material! Personally, I really liked "A History of Haskell: Being Lazy with Class"[2][3], partly because I like Haskell and partly because Simon Peyton Jones is such an engaging speaker.
[1]: http://research.ihost.com/hopl/HOPL.html
[2]: paper: http://haskell.cs.yale.edu/wp-content/uploads/2011/02/histor...
[3]: recorded talk: https://www.youtube.com/watch?v=3bjXGrycMhQ