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.
Green Tea Press are good guys, I would never feel sorry for paying them; they have done far more for humanity than my $30 ever will: http://greenteapress.com/
I'm a little confused about the various editions. Just a few weeks ago I bought the "Think Stats" ebook published by O'Reilly (same author). Is it all the same content?
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.