Do you annotate your books as you read them?

I did that with HS and college books, never "pleasure" books.  It would be a good idea though to help keep track of characters.   

NO!  I take care of my books.   On the other hand, I write things like 'metaphor' in the margins of ones I get from the library, and underline whole paragraphs in ballpoint pen.

no I don't. I just look up the difficult words and move on. 

Following in the footsteps of Sir Francis Bacon (some books are meant to be studied) and Ralph Waldo Emerson (active dialogue reading), yes, some.  

No - absolutely not! If you want to make notes as you read, do so in a separate notebook - NEVER in the book itself! That's what intelligent people do.  It's hugely arrogant and disrespectful to write in the book itself.

But whether or not you ruin a book by writing in it doesn't make you smart or stupid - that's just your level of intelligence.

No, I was brought up that it's desecration to write in a book.  I was so shocked when I started at a school where pupils UNDERLINED things in books with INK!  I still feel terrible just thinking about anyone doing that to a book.  The teacher made fun of me for being "precious" for making my notes on paper.  I'd leave little markers in the book and then write my notes in a notebook with the page number by each entry.  I'd even write out quotes from the book which I think was more useful than underlining when it came to exam time because I remember better what I write than what I passively read.  

These days when I read something I need to understand on an intense level I actually photocopy the relevant paper (it's mostly journal articles these days) in such a way as to end up with extra wide, clear margins on which I jot my thoughts as they pop up.  If I particular enjoy a passage in a book I mark it with a post-tab.  I can just about tolerate underlining in pencil, and I don't mind if the definition of a word is scribbled in a margin, but I despise heavy-handed permanent underlining or highlighting, especially by previous readers who seem to ponderously drag the pen or marker along every line as they read, seemingly just to guide their eye.  

Oh, and writing in the margins doesn't necessarily make the previous reader smart.  I buy a lot of used academic books and some previous readers are definitely as dumb as a sack of rocks going by what they write.  It's just something that's encouraged by certain teachers in close reading.  It doesn't mean that the book-mangler is either dumb or smart.

PS, I am also a total hypocrite because I LOVE marginalia in mediaeval manuscripts, not so much the traces of C19th scholars pouring kerosene on the vellum to "brighten" the ink though, cough William Skene, cough.

Yes, I do.  When buying a book, I actually like one that has notes and underlinings in it.  I can see what a previous reader thought.

Heck no, never!

I have way too much respect for the printed word format and its integrity. If I need to make a note, I would scribble it on a piece of paper and put that paper between the pages.

The only exception is if there is an error in the book that needs correcting.

True story: I once made the *mistake* of lending one of my horribly expensive text book to someone who did not have the respect I have for books, he (*&% ANNOTED IT with HIS garbage views. MY BOOK! Worth well over $100 (that was 30 years ago, imagine how much it must be worth inflation adjusted). I was MASSIVELY infuriated.

no, i dont do that

I do not write in the books. I write notes on sheets of paper that I keep with the book.