The “preview” step is a similar concept as described in (1.3.2.3) Grasp the overall picture roughly. First of all, you need to grasp the whole picture, instead of reading all the contents in detail.
This step is further divided into “survey” and “find the keywords.” In “survey” step, you collect information from the front cover, back cover, and table of contents. In “find the keywords” step, you open the book every 20 pages, find words which look important, and record them.
The time to preview is about 5 minutes per book. After this rough information gathering, you ask yourself whether to read this book. Or, you ask whether you change the purpose set in ❶ Preparation.