Day 4 - a book you lent out once, never got back and miss
Hmmm, this is a tough one! Either I have trustworthy friends or I don't lend out my books enough! I did lend Twilight to a friend who took ages to read it, then my daughter wanted to read it so I bought another copy for ... and then of course the friend returned the first copy!
Day 5 - a book you've read the most times.
This would be the entire Harry Potter series, but specifically I suppose the first one has been read the most, because it's the first and I had to re-read them all in order every time a new one was published!