If you are preparing for IIT JAM, there are two ways you can study.
By going for different books of different subjects.
By going for a complete study material.
The choice depends on you. The problems with going for different books is that there are no exam related specific books for IIT JAM. You have to go for standard books and have to study the complete matter. Now its good for your knowledge but if you want to study topics which comes in IIT JAM, it will be a waste of time.
So, it will be better if you can go for a good complete study material which is specifically designed for IIT JAM only.
Now the question arises which study material to go for.
With lots of options available where every institute claims itself to be the best, it is very confusing for an IIT JAM aspirant to choose the one who really the better one from others. I will suggest you to do a proper research on the internet about the top institutes like the above two and another institute which is popular among IIT JAM aspirant which is Eduncle. You can also check a review which I read: