In this case the set of sections that come before the body of the book are known as the front matter.
A Guide To Introduction
When the book is divided into numbered chapters, by convention the introduction and any other front-matter sections are unnumbered.
Keeping the concept of the introduction the same, different documents have different styles to introduce the written text.
If a Userguide is written, the introduction is about the product. In a report, the introduction gives a summary about the report contents.