I have other book related highlight questions but I will tackle them one post at a time.
Just writing them here for my own reference.
how to import highlights in Apple Books into DT?
how does one use the DT notes on book highlights to generate references for online articles and newsletter? anyone got a good workflow that’s high on convenience?
After HTML file of highlights is created, say on the ~/Desktop, drag and drop into the DEVONthink target database/group. Or use Menu: File → Import → Files and Folders, then pick that HTML file.
What have you tried that did not work as you expected?
I don’t understand question 2 of your extra questions so you might wish to expand on that, but for question 1 it’s not easy to export highlights from Apple Books so you might want to research that first as there are some workarounds and all are differing levels of annoying. Apple Books doesn’t play nicely. (My personal view is if you’re serious about reading and removing your highlights, avoid Apple Books because messing about with their thoughtless architecture will get old real quick. Obviously that doesn’t help much for highlights you already have trapped in their ecosystem. If you’re only talking about occasionally needing to remove notes, you may not mind.)
For your title question, importing a html file of Kindle Highlights, it’s no different to importing any other file type into DT, but I’d first ask if you actually want to store your Kindle highlights as an html file. I dont store anything as html myself, but this is very much a personal choice and many people do. If it was me, I’d want to store my Kindle notes in the same format as the other note files I work with (my notes are mostly in markdown with some rtf).