Which books and topics from class 11 and 12 are useful in a BTech?

School education is the foundation for your castle of B.Tech and M.Tech and PhD. So the more thorough you are in your fundamental subjects, the more successful you will be in your higher education.

If you have prepared for your JEE and other entrance exams well and succeeded, you don't need any school books for Physics, Chemistry and Mathematics in B.Tech because every university prescribes their own books and it will be available in your college library. If not, they will be available in the market.

But I personally never ever recommend any body to throw away or sell books to paperwala. One should always keep the basic text books for reference. So which ever book you had for Physics, Chemistry and Mathematics in 11th and 12th you should keep it with you.

In the first year of B.Tech, apart from engineering subjects, you will have Engineering Mathematics, Engineering Physics, and Engineering Chemistry. You can keep the following 11th and 12th books just for reference:-

Physics - NCERT book, Pradeep, S.L.Arora, H.C.Verma books.
Chemistry - NCERT is the bible for chemistry. and All in One by Arihant is also good.
Mathematics - R.D.Sharma

And when you are undergoing your B.Tech, you will come to know about the subjects and accordingly you can revise the topics.
All the best for your B.Tech.