Skimming or pre-reading a book is always a good idea; it is necessary when you do not know, as is often the case, whether the book you have in hand is worth reading carefully. You will find that out by skimming it. It is generally desirable to skim even a book that you intend to read carefully, to get some idea of its form and structure.
Finally, do not try to understand every word or page of a difficult book the first time through. This is the most important rule of all; it is the essence of inspectional reading. Do not be afraid to be, or to seem to be, superficial. Race through even the hardest book. You will then be prepared to read it well the second time.