Van Fraasen rejects both the old empiricism (sense data, Hume's impressions and ideas, etc.) and all forms of metaphysics (he singles out materialism particularly). For him, empirical objects are things like tables and people and stars. Science, not metaphysics, gives the best explanation of them, but it is a science that is not closed (ala materialism) with regard to what kinds of experience count. Although he does not elaborate, he seemed to include religious experience and is known to be a theist.
But it is not his particular set of philosophical views that makes this book excellent. It is the author's honest, robust, smart, open-minded thinking that makes it so good a read. He is simply good philosophical company and, for this, I recommend it highly.