It's a humble bundle so it would have been unusual for print books. The manufacturing cost is quite high for print on demand vs essentially free for digital do

It's a humble bundle so it would have been unusual for print books. The manufacturing cost is quite high for print on demand vs essentially free for digital download. But I am with you on the print books. I've spend many thousands of dollars building up my home CS library. Nothing helps me learn better than stepping away from the screens and reading paper in a comfortable chair!