You pick the document in Google’s own chooser, and only that one is shared with this app. Nothing to install.
Which tabs?
Unticking one stops it being fetched. The pages it already put in the book stay where they are.
From files on this computer
or drop .docx files here
They stay on your drive — each one becomes its own section, and Update re-reads them all.
Alternatively, connect through Apps Script about five minutes, and then it updates itself
In your Google Doc: Extensions → Apps Script.
Delete the stub, paste the whole script in
In the script, set SHARED_SECRET to a password you make up. (Run → gen makes one; copy it from the Execution log.)
Save. Then Run → test_localFetch and approve the consent screen — that is what gives the script access to your Doc. The log shows the title and a chapter count.
Deploy → New deployment → Web app: Execute as Me, access Anyone with the link. Copy the URL that ends in /exec.
Paste that URL and the same password below, then Fetch.
After editing the script later: Deploy → Manage deployments → pencil → New version. Saving alone changes nothing the URL serves.