The book is now available as an e-book, too.
In a way I'm still unhappy that it is not available as a free PDF. But on the other side that would be unfair to the amount of work invested by the publisher. It actually helps a technical guy like me to get the message across if there are people knowledgeable about writing helping in the process. I'm at least not that much into code that I am unable to judge the quality difference between what I can write and what I got back from OpenSourcePress.
Still, I retain the hope that it will be as free as the software I write at some point in the future.