There's few institutions like Astounding Science Fiction or editors like John W. Campbell Jr. Together, they are probably responsible for much of the tone and content of the science fiction genre in its formative years. Thus, during the tumultuous years of the 1960s, the changes to the magazine are an interesting example of how science fiction was changing: shedding one image and adapting, while positioning itself for the future.
The story of how Astounding Science Fiction became Analog Science Fact and Fiction is an interesting one, not only for what they changed, but what they didn't change. I think that the changes and non-changes are part of the reason for why they're still around today.
- Transformations: The Story of the Science-Fiction Magazines, 1950-1970, Mike Ashley. As always, Ashley's work is in depth and detailed, and he provides some good background information on to how and why these changes came about.
- I, Asimov, Isaac Asimov. Asimov has a couple of interesting words about Campbell towards the end of his life.
- A Requiem For Astounding, Ava Rogers. This is a fannish book dedicated to the legacy of Astounding, which has some good information about the magazine and its history.
- Astounding / Analog, Encyclopedia of Science Fiction. As always, there's a wealth of good information at the SFE, about Astounding AND Analog.
- Analog Website. Analog's own history has some good revelations.