When Kevin Kelly first began publishing a book version True Films, a collection of reviews of the best documentary films and TV programs featured his website, he offered a self-published print edition, then went the Lulu.com route for a more expensive color edition, and then a $2 PDF download. But for the third edition, which he's expanded to cover 200 documentaries, Kelly has prepared a free PDF download supported by advertising—but the ads will only be visible if the reader chooses to see them.

"Because the PDF file must reach out from your computer to the Adobe server to get the ads, an action that some readers may not approve, seeing the ads is an opt-in default," Kelly explains. "You have to agree to see the ads before any will show up. You will also need the latest version of Acrobat Reader (8) to see them. If you use an older version no ads will show up, and you'll see only the free book. Since the ads are adjacent to the book, whether you see ads or not will not affect the design of the book itself."

"My hunch is that books-supported-by-ads is one way to extend the FREE," Kelly says of the move, adding that he'd like to continue developing more free books, "outside of big publishing," if the model proves successful. His Wired colleague Chris Anderson isn't convinced. "Five cents a click isn't much when you think that only one in a hundred readers will click on an ad," he critiques, "and that these days most people don't have Acrobat 8 yet and won't even see the ads in the first place. But money wasn't why Kevin wrote the book, and I suspect he's more interested in the experiment than the revenues." I agree that the experiment seems worthwhile; that's why I'm attempting to register with the Yahoo! Publishers Network, which is supplying the ads for Adobe's program, to see if my modernized "interpretation" of the Tao Te Ching, which gets over 12,000 downloads a year in its PDF format, will qualify for inclusion. (Until now, the sole opportunity for financial compensation was a PayPal button; while I'm grateful to the handful of readers who have chosen to contribute, the effect has been minimal.) Wish me (and Kelly) luck—you might be following our lead!