Here are some of the books he has written under the John Herbert Charles Pippy pseudonym:
* What If? (1975)
* What If? II (1976)
* What If? III (1978)
* What If? IV (1980)
* What If? V (1982)
* What If? VI (1984)
* What If? VII (1986)
* What If? VIII (1988)
* What If? IX (1990)
* What If? X (1992)
* What If? XI (1994)
* What If? XII (1996)
* What If? XIII (1998)
* What If? XIV (2000)
* What If? XV (2002)
* What If? XVI (2004)
* What If? XVII (2006)
* What If? XVIII (2008)
* What If? XIX (2010)
* What If? XX (2012)
* What If? XXI (2014)
* What If? XXII (2016)
* What If? XXIII (2018)
* What If? XXIV (2020)
Colombo has also written extensively under his own name, but the "What If" series is his most famous work.