The variants give the game away. A genuine quotation tends to have one settled form; this one has never had one. The 1954 paraphrase reads “plan a book…washing the dishes.” The 1955 filler reads “planning a book…doing the dishes.” The version most often shared now — “plan a book…doing the dishes” — is a hybrid matching neither. Wording that keeps drifting is wording nobody is copying from a fixed original.
Watch out for the confident source. Because the line sounds like memoir, it is sometimes waved toward Christie’s An Autobiography ↗ — written between 1950 and 1965 and published posthumously in 1977. We make no claim either way: we were unable to search that text directly, and we will not assert a source we haven’t read. What we can say is that no one who quotes the line cites a page for it, and the earliest trail runs through newspapers, not books.
The bathtub is the better story anyway. The detail the aphorism sanded off is the one worth keeping: an antiquated tub with a wide mahogany ledge, and Christie sitting in it lining the rim with apple cores. She reportedly worked out the play Spider’s Web at the sink. The tidy version is more quotable; the real one is more interesting.