A book is never finished, it is abandoned.

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)
    (Source)

Fowler was speaking about publisher deadlines. Others have used similar phrases regarding the creative process as a whole. See Valéry and Abram.