Some time ago we changed our business model, and you can actually add new books for free in some of our writing modes!
For some other writing modes, you do have to subscribe to another plan. You'll see the plans at the bottom of the "create book" page here:
Here is an article from our Help Center about the different plans:
Please note the introduction of our new model did not have any affect on your previous book or books, which of course you still have full access to, regardless of your chosen writing mode.