Whenever you realize that you aren't enjoying the book you are reading, give it up. Remember reading shouldn't be a chore.
Set a reading goal.
Having a reading goal helps you figure out how much reading you need to do in a week or even a day.
Give yourself a deadline to complete each book.
Before you read each book, ask yourself when you need to complete this book by.
What I find interesting is that I tend to read books that I borrow from libraries faster than the books I bought. The reason is the books I bought don't have a due date! I don't need to return those books.
When you don't set a deadline to complete your book, there isn't a sense of urgency. And when something isn't urgent, you tend to procrastinate.