Mark Graban has written a great book on statistics, Measures of Success: React Less, Lead Better, Improve More. He focuses on Process Behavior Charts that help highlight the natural variability of a process and can really show when there has been a change in the underlying system. He does a nice job (for me) of simplifying the concepts and the underlying reasoning. He’s even put the math at the back of the book, in favor of providing clear reasoning and project flow.