After playing around with this for a while I found a fairly simple solution to this problem using Automator. It requires three separate keystrokes, however. First you have to copy the text in Books, using Command-C. Then create an Automator quick action that does the following three actions:
- Get Contents of Clipboard
- Filter Paragraphs to return paragraphs that begin with “
- Copy to Clipboard
I assigned the keystroke Command-Shift-I to this quick action.
Once it's all set up, you can copy a piece of text in Books, then run the quick action, then paste it wherever you like.
You would think it would be easier to make the first step of the quick action be "Copy to Clipboard" and then you wouldn't have to begin with Command-C; unfortunately, copying inside of Books doesn't seem to function normally. I even tried adding Command-C via the "record keystroke" method and it still produces wonky results. So, after wasting several hours trying to build the copy selection step into the workflow I decided to just accept it as a three step process: copy selection, trigger quick action, paste revised selection.
What you can do to streamline a bit, if you have Alfred installed on your computer, is create an additional Alfred Workflow triggered by a hotkey that triggers, in sequence, the following "dispatch key combo" events: Command-C, and then Command-Shift-I (or whatever keystroke you assigned to your Automator quick action.