Coaching classes cannot guarantee you an IIM seat. But even M S Dhoni needs a batting coach; so would a good coach help to get the best out of you or not has to be decided by you.
By joining a coaching class, what you would be getting is proper direction in terms of preparation and readymade material prepared exactly as per the CAT requirements.
There is no one single ideal number of hours that one should prepare. It all depends on your ability and the state of your preparedness. If you start your preparation early, about an hour a day would give you a good start. Similarly, when you start say in May/June, then about two hours per day may be required. And if you start in August / September, then even four hours a day may be less.
Though there are enough books written on CAT, there is no one — or even two — book that can give you an overall idea about CAT or the preparation for it. Some books give the CAT papers of the last 10 years, which may be useful to browse and understand the level of difficulty of the exam.