On any given day, I'm reading a few books at the same time. I do this for two reasons: weight and mood.
First, weight. I feel like I'm not myself if I travel without a book. This traveling can be as simple as the fifteen minute walk to work. Now, I know that unless a book is so interesting that it practically burns my hand if I put it down, I'm not going to read and walk simultaneously. I rarely leave for lunch, and when I do, it's with my colleagues. I don't have time to read at my desk. After work, I walk home. There is no time for reading during the normal business day. Yet, I am always with book. Shlepping around a hardcover is tedious. Hardcovers generally don't fit into my cute (and small) bags and are heavy. So, even if I'm reading a hardcover book, I need to also be reading a softcover or smaller hardcover for my daily travels.
And mood. To me, books are like friends. I'm interested in the characters and ideas placed in front of me. But, sometimes I get distracted. Sometimes, a book goes on and on, rambling like a senile turtle. Sometimes, a book is morose and determined not to see anything optimistically. And sometimes, a book is just plain dull. I rarely give up on books; however, I'm not above putting one down to replace it with one better suited to my mood. I'll come back to it.