Addendum to rule 1: Browse through the bibliography. Quick but effective heuristic to gauge how serious the book is, and how well the author knows his field. I

Addendum to rule 1: Browse through the bibliography. Quick but effective heuristic to gauge how serious the book is, and how well the author knows his field. If there isn't one, don't bother buying it. If it's a 20 page list of citations from reputable journals (or original sources, if you're reading a history book), then the author doesn't have to be a professor to be believable.