PDF books are the state of the art. When I go to game cons, almost every table has 2/3 of the players with some type of laptop, rather than carry a suitcase of books that are very heavy. I, grudgingly, made the jump before christmas, and bought an inexpensive ASUS netbook to use at Cons.
Typically, players will still want books though, at home. Usually this is a Core rulebook or other important reference book that gets used a lot.
Paizo's publishing model is very well suited to this. If you subscribe to a line of books for their Pathfinder RPG (say, Core rulebooks, or supplement books, or adventure books, etc), the company gets a direct sale of the book (at retail price, rather than at a lower price through a distributer), and gives you a PDF of the book for free. They are also VERY good about keeping the books erratta'd and updated, so the new changes are incorporated into the book, not just a 2 page file with the listed changes.
If you don't subscribe to the books, you can buy the pdf without getting the physical product, it is watermarked for security, and is sold at a discount compared to the printed book price. They also track which ones you have bought, so you can re-download the pdf at any time. Lose your hard drive? no prob.
I enjoy the printed books, and I enjoy the PDFs. both serve me well, depending on whether I am hosting a game, or travelling to a game, or at home rolling up a character or writing my own adventure scenario.
For the publishier, the hard question is whether or not to fufill both those desires of the customers, and how to implement it if they choose to do so.