On my Macbook, I have the Kindle app open with the book, as well as the markdown file open in a text editor (usually Sublime Text or MacVim, but it doesn't matter). When I read something that I want to summarize, I mentally commit it to memory and Command-Tab over to the text editor and type it out in Markdown. I don't copy-and-paste, because I find this solution allows me to retain the information better, and also it allows me to restructure the sentence if needed (usually editing for length). Usually my memory is not good enough and I Command-Tab back and forth, re-reading it in the Kindle app, and then re-reading what I typed out. No highlights or any fancy exporting. Sorry if that's anti-climactic.