Basically, you can read the whole book and then decide if you liked it enough to pay for it. Given the nature of the experiment, you can also consider paying if you felt that even if it’s not “your kind of book”, it was well done, with a sufficient mix of creativity, hard work and craft, to merit payment.
So: I’m not saying “pay if you think this is the best book you ever read of its kind”–because when you read a really awesome book, it’s easy to pay for it. No–I’m asking for honesty, based on your book buying habits: is this book on par with books that you have recently purchased and read? Another way to think of it is: you go to your community bookstore that’s cash only, go to buy a book and find out you don’t have the cash. The bookseller says, “look, take the book, and pay me next time.” It’s just putting the payment at a different point in the process. If you hated the book, you know your bookseller friend will take it back so long as it’s not trashed. Or, you could lie and say you hated it, thereby having garnered a free read.
Me, I would tend to pay the bookseller. But that’s just me. Again, it’s on the honour system–and unlike that friendly neighborhood bookseller, with whom you have a chatty, business relationship that allows him to trust you to pay next time, you don’t know me and don’t have that connection with me. So it becomes about the book–the work itself and your valuation of it–not your relationship with the seller.
Now, if you’re someone who isn’t comfortable with that and who likes to settle your debts up front, then by all means pay at the same time as you download–it won’t skew the experiment and will, in fact, play right into it, because it will be an indicator of your valuation of a given creative work. Similarly, feel free to pay at any point while you’re reading the book–I don’t require that you finish it before you ante up.