Even if the author wrote the preface, it was written after the book was written. You should read it after you read the book. If at all.

Exceptions: in textbooks, the preface often contains a description of what the book is about, what order topics should be studied in ("for a one semester course, do chapters 1-6, 10 and 14, for a two semester course, just do them all in order"), what knowledge is assumed, why you should use this book instead of the warhorse everyone else uses, and so on. Worthwhile stuff to see.0