A good book brings insight, understanding, surprise and delight. Non-fiction is based on fact not ideology. It informs us about our shared experience of the world. So, a book tells us what we want to hear is not a good book.
Style is also important.
Style is our experience of reading a book. Style speaks to what is important in life and how we perceive the world around us. Clear, lyrical style is a pleasure. Ponderous catalogues of abstract or cumbersome words are unpleasant when found in either fiction or non-fiction. You know a good book when you find one, and that's enough.