Keep the lines you mark.
Photograph a page you've marked in a book. Xcerpt keeps exactly the words you marked, with the book they came from, and brings them back to you.
Underline, highlight or bracket a passage as you always do. Take a photo, and Xcerpt saves exactly the words you marked, not the whole page, in a library organised by book.
Xcerpt is in testing on Android. It's coming to Google Play soon.