What is a six book series called?

There isn't a specific, universally accepted term for a six-book series. However, you could use some of these options depending on the context:

General terms:

* Six-book series (most straightforward)

* Hexalogy (using Greek for "six")

* Hexaptych (meaning "sixfold")

* Six-part saga

* Six-volume cycle

More descriptive terms:

* The [Theme] Chronicles (e.g., The Space Chronicles)

* The [Character] Saga (e.g., The Dragon Saga)

* The [Setting] Chronicles (e.g., The Atlantis Chronicles)

Ultimately, the best term for your six-book series will depend on the specific content and how you want to position it.