It's always better to join a coaching centre they have years of experience in training students and you will definitely profit from the way they get you into a pattern of processes to crack CAT. Plus by joining a coaching centre like TIME or CL or Endeavour you will get their mock series which are basically Chapter wise and Overall test papers which is a simulation of the real CAT exam. You will understand your strengths and weaknesses and work alot better on them if you become a part of it. Plus you'd be able to see how you perform in comparison to other students all over India.

Arun Sharma is a very good start for all sections but for advanced learning you should prefer joining a coaching class.

All the Best for your preparations.

SMIT-IIM-AMRITSAR-PGP/04