You get five of their PDF book editions (not protected, so printable, copy-and-pasteable, etc., but customised with the customer's name) for the price of one, and they donate every penny to the Australian Red Cross's Bushfire Appeal.
I already paid full price for a couple of their PDFs, and I thought they were great, so I just stumped up my $30(US) and got hold of another five. Definitely worth checking if you're interested in learning about web stuff. And maybe you can help them in the direction of the $100,000 donation they're aiming for...