He continued writing television scripts and science fiction novels, of which the most well-known are The Man Who Folded Himself (1973), about a man who uses a time machine to populate the whole of human history with duplicates of himself, and When H.A.R.L.I.E Was One (1972), the story of an artificial intelligence's relationship with his creators. WHWO was nominated for best novel for both the Hugo Award and the Nebula Award. A revised edition, entitled When H.A.R.L.I.E. Was One, Release 2.0, was published in 1988, incorporating new insights in computer science.
He is also the author of the War Against the Chtorr series of books, beginning with A Matter for Men (1983).