Can u suggest any book which is helpful to crack BIITs??

I'm assuming you'll be appearing for BITSAT 2018?
In that case, if you're preparing for JEE Mains as well, consider you're preparation for BITSAT done, with a few more elements that I'll get to in a little while.
First, you must know that BITSAT tests your accuracy and speed more than depth of concepts in general. Now this doesn't mean you shouldn't be clear with your concepts, it's just that the questions in BITSAT will be of the same level as Jee Mains.
There will be exceptions, with a few questions being very tough, but you need to avoid such questions in order to save time.
Now, other than your jee mains prep, you need to mainly focus on a couple more things.
1. English- Practice papers are the key here. And improve your vocabulary. I'd recommend an App called Galvanise for this.
2. Chemistry- Now I'm mentioning it here, because, usually there are more than a couple questions from saccharides. Which you won't find in Jee mains.

As far as books are concerned, you should continue with your jee mains books. Remember, speed and accuracy for BITSAT. So practice accordingly.