Shown below are some recommended Six Sigma books that are commonly read by practitioners throughout their journey.
As you would expect there are a lot of resources available around this topic with varying degrees of quality. These books shown below are some of the more popular resources.
Six Sigma Certification
Six Sigma Modules
Green Belt Program (1,000+ Slides)
Cause & Effect Matrix
Central Limit Theorem
1-Way Anova Test
Correlation and Regression