You do not need to buy any additional books other than previous years question bank which is more than sufficient for BITSAT. If you are enrolled in any coaching institute then follow the teacher's advice and have faith in coaching study material. If you are doing self study then you need to buy some reference books to clear your concepts. Now trust me there are hundreds of books which you can buy but not needed. You should buy only those books which you find easy to understand and for that you can review that book online. I will also suggest you to arrange some coaching study material as they have organised material.