K-9 Mail 5.806
Chromebook 3 / ChromeOS Version 94.0.4606.124 (Official Build) (64-bit)
If I have missed something please direct me, but I have looked in K-9 Mail itself and in the online documentation website and have not seen what I need,
How do I open the address book / contacts list in K-9 so that I can examine it and add new entries or delete obsolete ones?
K-9 Mail doesn’t include address book functionality. It uses the system’s contact database. On ChromeOS that probably means your Google contacts. You should be able to edit the list under https://contacts.google.com
You can add the sender of a message to contacts by clicking the contact picture in K-9 Mail’s message view screen. Clicking the picture of an existing contact should display the entry and allow you to edit or delete it.