My state of mind has a huge influence over what I read, but I think I do it unconsciously, so in a way it's a good way to gauge how well I'm coping. In the past, when I had bad depression I could only read very light, happy-ever-after stuff. It was a sort of escapism into a different safe world. The stronger I am mentally, the more I tend to read challenging books. But even so, it's good to alternate between books that need a lot of concentration and those that are a fun read.
Books are a huge comfort to people and a good coping mechanism, so I don't think anyone should feel guilty about what ever they read at any given time