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Apple Books
Apple Books highlights can be imported into NewTwos, one list per book and one thing per highlight.
Apple Books is the best of the reading integrations, because Apple sets no export limit. Every highlight comes across in full, along with any note you attached to it. Kindle, by contrast, caps how much of each book you may export.
Before you start
You need a Mac and the NewTwos desktop app. Apple stores Books highlights in a database inside the Books app itself, and iPhone and iPad seal that away from every other app, so the import can only run on a Mac.
That is not as limiting as it sounds. Highlights sync between your Apple devices, so the ones you made on your iPhone are already on your Mac and come across too.
Import
Open Settings Advanced Apple Books in the desktop app and choose Import Apple Books highlights. You will see a preview listing every book and how many highlights it found. Uncheck any book you do not want, then confirm. Nothing is created until you approve.
The first time you run it, macOS may need you to grant NewTwos Full Disk Access before it can read your Books library. The screen offers a button that opens the right settings pane. Turn it on, quit NewTwos, reopen it, and import again.
What comes across
Highlight text, in full, grouped into a list per book. Any note you attached to a highlight lands in that thing's note.
On iPhone and iPad
There is no import. What you can do is copy highlights out of Books by hand: open a book's notes, select the highlights you want, copy, then paste them into Settings Advanced Apple Books in a browser.
This reads the book title from the boilerplate Books adds to the copied text, so it works best on an English device. It is a fallback, not a replacement. The Mac import is complete and exact.
Re-importing
Re-importing is safe. Every highlight carries an identifier from Apple Books, so a second run adds only what is new and skips what you already have. Editing a highlight in Books and re-importing updates nothing, it stays as first imported.