As you know by now, "Open Life: The Philosophy of Open Source" is available online and you can read, download and print it for no charge. If you want, you can also buy your own copy from a bookstore or online.
If you don't care to buy the paper version of this book, but have read the online version and would like to pay something, you may support the author by tipping. A tip is a small online donation. You can donate if you have a PayPal account or a credit card.
The amount to tip is up to you. I recommend as a rule of thumb to think about what you'd give to a street musician. I personally am a big fan of the musicians in Helsinki and always give generously :-)