If you are skimming a book/website/documents, you should read the blurb, the preface and table of contents. The Chapter headings provide a reliable indication of the content of the book. You can turn the pages randomly and check its readability, style and seriousness in terms of treatment. You can focus your attention on subheadings, bold, italicized or underlined text, diagrams, graphs, charts and photographs, if any. If there are chapter summaries, you get the vital points quite easily. You can go to the text to find the details of points in the summary you found interesting. Once you find that the book is worth detailed study, you can go for it.