Scan the title of the book and the author's name. Recall if you have read anything previously by this author. Consider if the title has any impact, or if it falls flat.
Read the preview once without pausing or considering the text. Get a sense of the emotional impact that the words have. Assess the author's style, and whether or not you want to read further.
Read the preview again. This time, read more slowly and pay attention to details. Determine if the characters or the hints of plot hold any special interest.
Ask yourself if, having now read the preview, you wish to continue reading. The answer to that question will determine whether to purchase the book, wait for it to show up at the local library, or disregard it altogether.