Firstly i would like to say please remove from you mind that your are from science background and CA course for you is difficult. CA-CPT is framed in such a manner that even a science background student can easily clear it. As for science student Accountancy may be difficult at the same time for commerce student Q.T may be difficult.
For books i suggest you to go through the ICAI material at once in detail. And along with that solve the modules provided by ICAI. I am sure by studying the material and solving modules you will be confident for writing exam. Along with this if you wish you need a book for reference then go for Paduka publications books, especially its model test papers book, they are superb..!!
For tips i just want to say answer the question by using logic and not by using formula. (Use formula until you understand the concept and after that as it consumes more time in exam) And try to prepare your own notes that you can refer before entering the exam hall. And importantly answer only those question of which you know the answer correctly.