What is Charles’s explanation of “white-hat books” and “black-hat books?”

From the text:

'Well, Jim - ' they perambulated, Dad running his fingers along the book spines - 'he wears the black ten-gallon hats and reads books to fit. Middle name's Moriarty, right, Jim? Any day now he'll move up from Fu Manchu to Machiavelli here - medium-size dark fedora. Or over along to Dr Faustus - extra large black Stetson. That leaves the white-hat boys to you, Will. Here's Gandhi. Next door is St Thomas. And on the next level, well…Buddha.'