What is the longest book series?

The longest book series ever published is "Perry Rhodan," a German science fiction series written by various authors under the pseudonym Clark Darlton. The series was first published in 1961 and continues to be published to this day, with over 3000 books released as of 2021.

"Perry Rhodan" follows the adventures of the titular character, Perry Rhodan, a fictional space hero who leads humanity into the distant future and explores the vast expanse of the universe. The series blends elements of science fiction, space opera, and adventure, and has garnered a dedicated fan base worldwide.

The sheer volume of books in the "Perry Rhodan" series makes it the longest book series in terms of number of works. The series has also been translated into multiple languages, further extending its reach and popularity.