priyabratg
First,you should go with gmat official guide.If you feel confident in solving these questions then you don't need to go for subject wise because level of difficulty is same and each question is a gem in all the books but you need to practise tougher questions because you will get tough ones in real GMAT.
The GMAT is an adaptive test, so it's not as if it will present test takers with only difficult questions. Even at the highest (score) levels, the GMAT does not present only super tough questions, and it's safe to say that OG questions are very representative of the actual exam. "Real" questions might
feel tougher because of test day pressure.