Good notes are critical when writing a book.
First, for simply gathering the information needed when it comes time to write the first draft.
Second, to prevent inadvertent plagiarism.
Third, to give accurate sources for your information and to create a good bibliography.
Here's an excellent summary of one style of note taking:
“How to Take Notes”
The objective of note taking is to come up with an outline for the chapter, paper, or book that you are taking notes about.
Before you start
1. Formulate the research question. Have it in mind as you read, listen to interview, etc.
How it’s done
1. Take copious notes. Figure 4-10 times as many as you’ll need.
2. Compress the material, and extract key points. Do not dwell on details.
3. Use (print to) 3 ring binder paper.
4. Number each note and sub level.
5. Source each note carefully.
6. Distinguish important points
7. Translate into your own words
Things to avoid
1. Copying lengthy chunks of books or web pages or interviews.
2. Jumbling topics together.
3. Writing prose.