A book is never finished, it is abandoned.

Gene Fowler
Gene Fowler (1890-1960) American journalist, author, and dramatist. [b. Eugene Devlan]
Quoted in H. Allen Smith, The Life and Legend of Gene Fowler, ch. 27 (1977)
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Fowler was speaking about publisher deadlines. Others have used similar phrases regarding the creative process as a whole. See Valéry and Abram.