Buy a book from Apress (such as the superb Definitive Guide to Django) and you can get the PDF version for a fiver, so you can keep a copy on your laptop when you are on the road.

What a great idea. I don't want only the PDF - I much prefer to read the dead tree edition. But you can't carry dozens of chunky tech books around with you, so I want the PDF too. Clearly I shouldn't have to pay full whack twice, but the fiver that Apress is asking seems fair.

I wish O'Reilly did this.