It sounds as though you are simply doing the same thing over and over again. I am reminded of the remark about the difference between someone who has fifteen years' experience and someone who has one year's experience fifteen times. Rather than piling up study texts, you should be working to identify trends in your performance. What types of questions do you consistently miss and why? What principles are you missing? What type of wrong answer are you falling for most often. Find a book such as Kaplan's that will give you not just answers, but detailed explanation of how to arrive at those answers and analyses of why the wrong answers are wrong.

Try keeping an error log. Analyze practice tests and problems you have missed and record the reasons behind your errors. Over time a pattern should emerge. Are you failing to read the question properly? Are you missing a certain type of grammar issue? Are you consistently falling for 180s or missing out-of-scope answers? Are you misinterpreting a reading passage? Once you find a recurring error, you can correct not a single problem, but a whole string of problems. None of the questions in any of your books will be on the test. You need to be absorbing the concepts behind the questions. Once you have the concepts firmly in your grasp, your score will improve.

Good luck,