Use a Dictionary

Keep a dictionary with you as you read a book. Look up any words you don't understand. You will not only better understand what the book says and means, but you'll know what the word means the next time you run across it.

Also, even if you think you know what the book means, it's sometimes worthwhile looking up some of those words anyway. The writer may be using the word in a different context from the one with which you are familiar.