Bit of a shameless plug but I've been thinking about this for a while and I recently built a free/open-source tool called Emdash [1] to tackle the problem. I’m a heavy ebook highlighter, but I found if I didn’t have a compelling way to review those highlights they fell out of my understanding. My tool uses on-device semantic embedding which allows you to peruse your collection and see related passages from other book…
This is a beautiful app - my problem with tools like this is I don't use a Kindle so ingesting highlights is painful, but I'll plan on giving this a shot nevertheless.