Also written in the 60s, it reflects upon their experience with how to convey uncertainty in prose, in a meaningful and consistent manner. Imagine an analyst writing a two pager for the President, who is going to use it to decide go/no go on a covert action. It's a big, ugly and complex situation - with a mixture of highly confident assessments, educated guesses and total speculation. Synthesizing it into something simpler is The Art of Writing; doing so while not losing the anecdotal quantification of uncertainty of the discrete issues is a more unique skill.
If you are in a position where you write about uncertain situations, it's worth the read. I'm a security guy and frequently find myself in that role (think annual risk assessments and estimating the likelihood of a successful attack). It's been a go-to reference for many years to refresh my thinking.