What Belanger says on page 43 is accurate, and it’s also a #DevilReference! So yes, the phrase is a relatively modern variant on the older idiom “God is in the details.” It isn’t exactly true that the phrase doesn’t appear in print prior to 1975—it was certainly known before then—but a search through Google Books only turns up six occurrences in the 25 years prior to 1975:
- An issue of Congressional Quarterly from 1956 (“He certainly understands a lot of the details and a lot of the policy issues, and with taxes the devil is in the details”).
- Newsweek 1965, 65(1): 173 (“’The devil is in the details,” Beitz says.”)
- A hearing on the House Committee on Appropriations from 1969 (“I found out long ago that the devil is in the details”)
- A Congressional hearing from 1972 (“But, as you and I know, the devil is in the details”)
- Socialist Commentary, 1973, p. 26.
- The Congressional Record from 1974 (“The devil is in the details.”)
|This Google Ngram shows that the phrase "devil is in the details"was relatively uncommon prior to 1975,|
and only surged in popularity since the 1990s.
Granted, the texts digitized by Google aren't necessarily representative of all published books. But Google is surprisingly thorough, and a fairly good real-world indicator. Good enough for fiction, at least.
And yes, I actually researched all of this—including Flaubert and Decadent literature—just to write one paragraph in The Billionth Monkey. Who says writing fiction is easy? ;)