I really don't like to write in books, unless it's something for a class (in which case I figure whoever reads it after me can benefit from my wisdom

), but occasionally I'll write in some comments if there is something about violas/violists or some subject I'm really into.
At the moment I lack the free time to really get into a book seriously, but I usually try to have 2 or 3 books going at once. It's mostly because I have so many books that I haven't gotten around to reading (somewhere around 200 novels), so I start up a few and eventually pare it down to the one that holds the most interest for the longest amount of time. Then I'll go back and pick up one of the others after I'm finished with the "best" one.
As for speed, I'm a pretty fast reader if the novel is really good. I normally finish off most books that are 300-275 pages in a week, given enough free time -- but lately I'm taking forever even for a weak little 200-pager.
Jam it back in, in the dark.