I am super into having a pen while reading. There is something about being able to write in the margins, underline things and interact with something while reading.

But...

Second hand books with underlines....nope.

Authors/editors that put them in for me....double nope.

The best is coming back to what you wrote years later and having the "I was an idiot" reaction. Watching your interpretation of the world change over time is nearly as good as getting to interact in the first place.