There are obvious forms of book mutilation, then there are things that others consider to be "book mutilation" that can be controversial. Obvious forms include tearing out pages, destroying the cover, etc. Then, there are the not so obvious. These include throwing away dust jackets, highlighting, and writing in the margins.
I recently decided that I like how my hardcovers look when they are not in their dust jackets. This sparked something in me to want to see what other people think of throwing away, or even taking off their dust jackets. Many people say they take their dust jackets off to read, then put them back on to shelve them. Others threw them away because, like me, they liked how their books looked without the dust jackets. I hadn't realized that there are people who consider throwing dust jackets away to be "book mutilation". I do not agree with this. I like my hardcovers to have an antiquarian look 9 times out of 10. If the dust jacket is something I love, I may keep it on. Otherwise, I feel as if over time, the dust jacket will just get dirty, get torn, or get that little lip on the bottom for sitting on the shelf for so long. I obviously believe that it is everyone's own preference, but people who decide they like how their book looks without the colorful dust jacket (opting for the mildly uniform, vintage look of the spine and blank cover) should not be scolded for doing so.
With highlighting and writing in margins, I think people are allowed to do whatever they want while they're reading so they can remember certain quotes or write notes to remember for later if they are in a discussion group, or even if it helps them understand a book better in general. Too many people are worried about keeping books in mint condition when, to me, that is not the point of owning books. I am working to perfect my personal library at home, and I would never dream of letting someone borrow a book for too long; much less sell one. They are very precious to me. I read how I want to read, and yes, that does include dog-earing the pages.
If you are mainly worried about being able to sell your books later on, be my guest and keep them in mint condition as well as you can. As for me, I'll make sure to enjoy my books.