[AllenDowney]

Sorry! It's really not meant to be a trick. The book is under a CC license. I provide the PDF version at Green Tea Press, the LaTeX source for anyone who wants to make a modified version, and a not very good HTML version (some of the math is broken). O'Reilly provides the printed and Kindle versions.

Right now all versions have the same content, but I will continue to revise, so you can think of the version on Green Tea Press as the draft of the second edition.

I know it can be confusing, but I hope the benefits of the free license make up for it.

[lylejohnson]

Thanks for the clarification, Allen. It's a great tutorial, I was glad to pay for it, and I will be recommending it to others!