Apprenticeship is the only way, but you need books to help pass the exams.

Sacramento State courses were at one time considered the best: https://owp.csus.edu/operator-training/courses/correspondence-courses/wastewater-courses/course.php?sectionId=1947&has_child_courses=1

I'm even if they aren't considered great anymore, doing the correspondence course which costs $50 and is a series of online multiple choice questions gives you 90 training hours towards your renewal requirements which is more than two years worth here.

I understand operators these days prefer the WEF manuals: https://www.wef.org/publications/operator-training-manuals/ Perhaps they kept up with the standard exams better.

On the engineering side I love the WEF MOP, especially MOP 8: https://connect.wef.org/s/store#/store/browse/detail/a2nRP000000oa6jYAA

We joke that Metcalf and Eddy is our bible but it's the most basic starting point in research. I think people take the joke too literally sometimes:

https://www.mheducation.com/highered/product/wastewater-engineering-treatment-and-resource-recovery-metcalf-and-eddy.html?viewOption=student