Some cloud providers require us to have a privacy policy, so here it it.
All your base are belong to us. You have no chance to survive. Make your time. Ha Ha Ha.
Well, OK, that isn't our policy, but our real policy is almost as simple. Calibre Companion (CC) does not share any customer information with MultiPie or any other entity without the user taking explicit action. CC does not report to any entity, human or corporate, what books are in your library, when you connect, what you read, or how you categorize and manage your books.
However, CC has no control over other apps or systems you might use as a consequence of using CC. For example, connecting to your calibre library shares information with calibre. Reading a book shares information with the reader app. Putting your library in the cloud shares that information with the cloud provider, and accessing your information in the cloud shares that access information with the cloud provider.
A particular case: sending a CC debug log to MultiPie shares information with us about how you have been using CC. MultiPie will not use that information for any use other than to provide support to the user who sent the log and to fix bugs in CC. The debug logs are deleted as soon as the problem is resolved.