I am of the opinion that a home can almost never have too many books or too much art! One of my favorite styling tricks to hang one or a few pieces of art on top of the bookshelves themselves. I love the depth it gives a space and the eclectic, collected style it can bring. At my first installation of a big, custom project, I was so surprised to see my boss layer various paintings and mirrors over ceiling to floor bookcases - the result was really beautiful and unexpected. The key is to make sure the art covers less than half of the face of the bookcases and to secure it well!
Here are some lovely examples of layering art and books . . . .
Do you think you might try this look at home?