Maybe I misunderstand something, but the article text says:

> in 1992, the US Justice Department claimed releasing the third book could harm national security by revealing the NSA's "code-breaking prowess". It was finally released in December last year.

When we move to the first page of the linked book it says:

> This text constitutes the third in the series of six basic texts on the science of cryptanalytics.

So there is still book 4, 5 an 6?

(im probably on some list just for asking)