Pieper begins the book as almost a marxian (though also anti-communist and anti-totalitarian) tract and ends as an apologist for Christian philosophy (though not, necessarily, for Christianity the religion).
Part of this is that he writes as a German in the years immediately following World War II. Neither Stalin nor Hitler nor the devastation of war can be ignored. Leisure, he notes, seems a luxury in such times, when so much rebuilding is necessary. And, though he doesn’t explicitly say it (though I think it implied in the book), when so much recompense is necessary.
He rejects the idea of intellectual ‘work’ in favor of less loaded words. How is ‘work’ loaded? It is for him because he wants something that does not demand an outcome, as in the product of work. He wants something that reflects contemplation and wonder (and revelation? It naturally follows, though he eschews such gnostic language).
The obvious comparison is between ‘pure’ scientific research and ‘practical’ scientific research (which, as Pieper would no doubt be quick to point out, had he made the comparison, is founded upon the results of pure scientific research).
Ultimately, though, the title is really misleading. He is not advocating, in the end, for leisure, but for philosophizing as a vital part of life.
He most frequently cites Plato and Aquinas (which made me wish I knew more than broad strokes about his philosophy), but it is Heidegger who most clearly haunts him. He mentions him, but tries to avoid mentioning him (not unsurprising, considering the time when he was writing). Like Heidegger, he seeks a way of being in the world and this leisure, which is really philosophical contemplation and study, is his solution. But while Heidegger’s is nearly theological, Pieper’s is, in the final analysis, explicitly theological. Sort of. He doesn’t argue that Christianity is necessary for man, only existentially profitable, arguing, as it were, but not proselytizing.