Is there a sixth Daniel x book?

There is no official "Daniel x" book series. It's possible you're thinking of a different series, or perhaps a fan-fiction creation.

Could you please provide more context? For example:

* What is the genre of the book series you're thinking of? (e.g., fantasy, romance, thriller)

* Do you remember any specific details about the plot or characters?

* Is there a specific author or publisher you associate with this series?

With more information, I can help you find the book you're looking for.