Apologies for misreading your post.
Similarly when I selected borrowed by I have several people in there but I have never lent them a book at all. How can I delete them off the list? I can't find out where it's stored so I can't get rid of their names. It's not in the autofill preferences.
You should be able to "Return" those books.
1 - click on the 'Borrowed' collection in the Source column on the left. This will bring up a list of all the books that are currently borrowed.
2 - Select each book or CMD ⌘ click multiple books
3 - Right click > Mark as > Returned
OR
4 - Book > Borrowed History
5 - Highlight the Borrower and click on the minus (-) button bottom left of the panel
If you want to completely start afresh with the 'Borrowed' facility
1 - Right click on the 'Borrowed' collection in the left hand Source column
2 - Select 'Delete'
3 - To recreate a fresh empty 'Borrowed' collection: File > New Special Collection > Borrowed Collection
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As for your main question. There is three ways you could achieve this.
'1 - Last Read' facility
Book > Mark as >
2 - Excluded Collection
You could create a totally separate Collection which you could then exclude from the main Library collection
1 - File > New Collection (or New Collection from Selection)
2 - Give it a name, Amazon in this case
3 - Right click: Exclude Collection from Library
Now when you borrow an Amazon book add it to this Excluded Collection and if you have a mandatory Return Date then you can use the Borrow facility but just use your name only, that way you will get a reminder to return the book to Amazon.
OR
You could create an Excluded Collection and name it 'Borrowed'. Then create a Custom Text field 'Borrowed from' and a Custom Date field for the borrowed date and still be able to utilise the Borrow facility to remind you to return the book.
Anyway play around with this idea of an 'Excluded Collection'
3 - New Bookpedia Database.
1 - Ensure that Bookpedia is Quit in the Dock
2 - Restart Bookpedia by holding down the Option key at the same time
3 - Create a New Database
4 - Give it a name Bookpedia Borrowed
5 - At this point you could Create a New Folder and Location for your new Database - your call (personally I would keep them in the default ~/Library/Application Support/ .
Now each time you start Bookpedia after Quitting you will have to Hold down the Option key and Click the 'Choose database' button. NOTE: Bookpedia will start up on the last used Database if you do not use the Option key on start up.
PS. if you know a little bit of HTML coding you could get you hands dirty and write your own or adapt an existing Style (Info Template). Have a look here:
Extras and here:
Creating a Template
I hope all of this is of some help to your questions. Let us all know how you get on