* He wrote under different names: He published works under his own name (C.S. Lewis) and under the pseudonym "Clive Hamilton."
* Some works are part of series: The Narnia series, for example, is considered one overall series, but has multiple individual book titles.
To give you a good idea:
* For works under his own name: There are over 40 books, including novels, essays, poetry, children's fiction, and scholarly works.
* For works under "Clive Hamilton": There were only 3 books.
It's best to think about his output in terms of categories:
* The Chronicles of Narnia: 7 books
* Space Trilogy: 3 books
* Mere Christianity: 1 book
* The Screwtape Letters: 1 book
* Other works: Novels, essays, poetry, and scholarly works across many categories.
So, while it's hard to give a definitive number of "book titles," C.S. Lewis was a prolific writer with a vast and influential body of work.