Robert Caro says he interviewed many senators and Senate aides, and that an unspecified large share of what he learned was omitted from his books but preserved in his file cabinets.

“I wrote so much more, and I have so mu- I interviewed so many senators, so many assistants to senators, to try and write about the Senate.”

“And then I had to cut out or not write or even go into”

“the vast, I don't want to say 90%, but I don't really know what percent, of what I learned is not in the books. But they are in the file cabinets.”