best book for contract act

If you really want to study Indian Contract Act, 1872 in detail, then go for Textbook on Law of Contract and Specific Relief by Avtar Singh. People read this for Judiciary or PG exams.

Else, if you want to study for LL.B, then you can buy Indian Contract Act - R.K. Bangia. To be honest, Bangia is a pretty good book. I had read it for my Semester exams.