Start at Settings → Books & Shelves → Library. Connections says what the plugin can reach.

Calibre shows as off or not reachable

Why: the Content Server isn't running, or the address is wrong. Fix: in Calibre, select Connect/share → Start Content server. Then select Connect…, check the Content Server address and select Test.

The library reads but changes aren't written

Why: the Content Server doesn't allow this connection to make changes. The Library tab says Connected, read-only, and Connect… → Test fails the Changes check. Fix: in Calibre, Preferences → Sharing over the net → Advanced → Allow un-authenticated local connections to make changes, Apply, then stop and start the server. Or enter a User and Password that may make changes. See the settings Calibre needs.

The library is read-only

Why: the server is off, and the library is being read from its folder instead. Fix: start the Content Server. Until then, syncing notes works, and book files and writes wait.

A change shows as a conflict in the write queue

Why: the value changed in Calibre after the change was proposed, so it wasn't written. Fix: look at the book in Calibre. Reject the change, or look the book up again to propose a fresh one.

A sync lists books it couldn't place

Why: more than one note could be the book, or the match isn't certain. It lists these and doesn't guess. Fix: add the book's Calibre id or ISBN to the right note's properties, then sync again.

No covers come from Google Books

Why: Google gives no covers without an API key. Fix: add a Google Books API key on the Notes tab, or rely on Calibre and Open Library.

"This plugin needs Model Hub…"

Why: Model Hub is missing, turned off or too old. Fix: install, enable or update Model Hub. Syncing notes and covers works without it; shelves, summaries and lookups need it.

A job has made no progress for five minutes

Why: a call is slow or stuck. Fix: wait, or cancel it from Books & Shelves: Open the jobs view.

A run stopped at a limit

Why: a call was billed well above its listed price, or one of Model Hub's limits was reached. Fix: check Model Hub's usage view and the tier's limits, then run it again.

A pack isn't used

Why: its pack.yml has a mistake. A notice names the pack and the problem. Fix: correct the file and run Reload packs.

Questions

Does it change my Calibre library?

Only through the write queue, after you accept a change, and when you agree to an EPUB conversion. See Changes to Calibre, and undo.

Does it work on mobile?

No. Books & Shelves is desktop only for now.

Still stuck? See Getting help.