Book #71 is (in an even bigger surprise)
Skylark of Valeron, by E. E. Smith. This is the sequel to
Skylark Three, the name being that of a
new new spaceship constructed during the course of the book, after poor old
Skylark Three, still with that new car smell, is destroyed by the Intellectuals, creatures of pure mental energy.
Our hero's only escape by rotating the original
Skylark
into the fourth dimension. Upon returning to normal space, after
defeating the evil Hypers, they save the Valeronian's from the evil
Chlorans, and build the Skylark of Valeron (our hero's wife objecting
to the name
Skylark Four). Skylark of Valeron is a sphere again, but now a thousand miles in diameter.
This is either the
evil Intellectuals or the
evil Chlorans attacking poor
Skylark Three.