In statement of purpose, good to mention about learning from some professor's lecture notes and books? In statement of purpose for applying to a PhD program, is it good to mention about learning from some professor's lecture notes and books (not papers, because don't get to read them yet), and finding them helpful? 
Will it make the statement more personal and therefore good?

Or is it better not to say so, and rather to keep the essay short?
You do not want to convey the message, "I am applying to your school because I think Professor X is great."  Everybody knows Professor X is great.  That's why they hired her.

The question is, why should they want you?

A message you do want to convey is, "I am applying to your school because my research goals are well-aligned with existing interests in the department."  If you can incorporate Professor X's notes and books into that story, great.  Otherwise, leave them out of it.