6 series starting with X. Page 1 of 1.
X-Factor
2 books · 1986
X-Men Annual
3 books · 1970–1979
X-Men: The Trial of Magneto
5 books · 2021
Xanth
Piers Anthony · 47 books · 1977–2022
Xeelee Sequence
Stephen Baxter · 11 books · 1991–2018
xkcd
Randall Munroe · 3200 books · 2006–2026