I attended a one-day seminar of Tufte's one time and agree that it was pretty good. But he was by no means "giving away" his books; they were built into the (considerable) price of the event. I also recall being amazed at how many people showed up. This was in a large hotel ballroom in SF and we were jam packed in there.

Another fun fact: when he couldn't get his first book (VDQI) published the way he wanted, he mortgaged his house and published it himself. Respect.