E-books most of the time and I’ve been reading on phone screens even before they became smart-ish. I still remember the joy of carrying books in the pocket, even if the screen of the Nokia phone I was using at that time was pretty small and books had to be in a very specific text format.

Paper books are fantastic and cannot be replaced when they are used as manuals. The memory of a printed page with nice graphics and of its relative positioning in a technical or scientific book that is read more than once is not replaceable by an electronic equivalent.

Finally, my sight does not improve over time and e-book custom fonts and size can be very helpful!