In order to work and use your time effectively I would advise you to take a closer look on the table of content, providing there is one.
Read the first paragraphs of a text. That gives you a clue of what the text is about.
Besides, it's important to make sure that the author and the text fit your topic. Try to get information about both, afterwards you can decide whether the author provides a useful source. There are authors that are more important and popular than others.
Make sure that you do not lose the thread. That is what happens when you have a lot of books (and other sources) and cannot see the wood for the trees anymore.