You could structure it by section/chapter depending on the length of the book, espcially if you're writing a book that will have over 20,000 words and cover different, unrelated topics. Each section should be about a general category or topic. The chapters contained within the sections should be related to that particular section.
For example, suppose I were to write a book on black hat. I could structure it like this:
Section 1 Blackhat SEO
chapter 1: old seo methods
chapter 2: new seo methods
chapter 3: other methods
Section 2 Blackhat Affiliate
chapter 1: the long affiliate way
chapter 2: the easy affiliate way (cookie stuffing)
chapter 3: other blackhat affiliate methods
Section 3 Blackhat Money Guides
chapter 1: traditional money methods
chapter 2: innovative money methods
chapter 3: other money methods
If you're book is not long at all, simply structure it chapter by chapter with no sections.
Hope that helps!