One sidestory that I found particularly intriguing was a note between, I think, McNamara and Lyndon Johnson, in the early 60s. In it they discuss the budget surplusses they are expecting for the late '60s, and they fear Congress will call for tax reductions before they can use the surplusses for their Great Society plans. So in the meantime they think Apollo a good and popular method to keep the budget balanced until they have better things to do with the money. Then came Vietnam...

But more on topic, the whole 'spin-off' concept seems pretty much invented by NASA in later years to justify their budgets, and it is used for so many 'big science & engineering' projects when the wider public has doubts about the costs.