I haven't been much of an underliner. I have made notes in a separate notebook of things I wanted to remember and thought I might need help. My picture of a book with underlinings or highlights in it is one of scattered bits that would need to be found and copied. A Kindle makes it easy and fast to highlight a passage and leave that remark in a downloadable file and/or share it on Twitter, Facebook or Goodreads.
I got my first Kindle in 2008. When I got the Kindle, we gave away lots of paper books but I am confident that few of them had much underlining or highlighting. I assume the act of highlighting is usually to put the underlined material more firmly in mind.
I looked through my books to find one that might have selected passages. I settled on "Paleofantasy" by Dr. Marlene Zuk, a book included in a talk I gave recently. Here is a link to my highlights from the book.
I note that it is easy to find the highlights for a given book but no tool or procedure has been included to find the highlights first and then the book. That is what I wanted to do when I asked what book was marked up the most. I couldn't find out easily. The highlights skip large parts of the book and I don't feel I can say whether my knowledge equals what is marked up. When I read a chapter over, there were things I knew already and that I recognized.
Later, I made some notes for my talk and I haven't compared those with the notes from the Kindle file while reading the book. I usually feel that I know when I come across a new idea of importance and it is the newness, the novelty, the value of the idea or the cleverness of the expression that grips me. I am interested in situations where students or other readers are so tense or exhorted to know and remember what they read that their tension interferes with full, deep, easy comprehension. I like the idea that good reading is a internal conversation between the reader and the text.