Before I started writing, I never spent much time reading a book’s Preface or other front matter, I just skipped right into the book and went from there. After writing my share of books for a variety of skill levels, I find the Preface and other such introductory material to be of great importance. The Preface can be used to tell the reader what skill level they need in order to read the book and what they can expect in the book.
After reading some reviews, it’s clear there are a lot of people who don’t read the introductory material. If they did, they’d find out that a book is above their skill level (or below their skill level in some cases). My favorite example is a review for a beginner-level programming book. By beginner-level, I mean someone who has never programmed anything before, in any language, anywhere. The book’s introductory material makes it clear that the book is for newbies and only newbies. However, the reviewer rates the book poorly because it doesn’t contain a certain type of programming structure. Obviously, if the reviewer knew programming structures, there’s a great chance they were well beyond a beginner-level book already!
In any event, introductory material is indeed important. I needed to write several books to realize that; hopefully others won’t have to write books to extract the same lesson.