I earn zero revenue from any sales of this book.
However, I prefer holding a book, & reading a physical book offers greater comprehension and retention. It's listed at the author price of $6.48 + tax/shipping, NO revenue mark-up, so I can print on-demand for myself, & students can afford it who prefer to read a book. This lets anyone have the option without having to expend 300+ pages of ink printing the PDF (ink ain't cheap!).