I'm studying in grade 7 and I have a deep interest in science and mathematics. But the sums and questions asked her aren't like like the ones which are taught in my school or I know how to solve especially the practise questions. I just want a good reference book which I can refer so I understand the basics and solve the sums. It would be better if it's a eBook or the book is on sale on a well known website like Amazon or eBay. (I don't care about the cost. Also it must have a bit basic revision before starting the difficult sums as I already told you I'm in grade 7).