If you allow it, Tenthleaf saves cookies (small items of data) onto your computer to indicate that you have unlocked the different chapters of the book. It can only do this if you have cookies enabled.
This will be announced nearer the launch date.
The Tenth Leaf is highly graphical and uses some of the latest web technologies to provide a richly featured environment. I recommend that you download the latest version of your favorite web browser so that you can take full advantage of these features.
If you have unlocked several chapters on one computer, you will be able to submit the unlock code of the last chapter using a form that will be available from the Treasure hunt start page. All earlier chapters will then be unlocked.