As of macOS Catalina (10.15), Apple no longer supports the Ink framework that inkBook relies on.
You can continue to use inkBook on previous version of macOS, now for free using the following registration.

To register inkBook 2, copy the contents of this email (Edit menu -> Select All, Copy) and open inkBook 2.

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Name:
"Free"

Registration Key:
"F55F-C46B-B3D1-85FA-9255-F402-DD5A-B2DC"
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