res sir,

which reference book should i buy for class 9 maths

@ class 
in class 9 u just need to take ncert for your revision . 
class 9 is just a foundation , and ncert is your main textbook , and for iit foundation u can take the books of mtg publication as well , and there are books avaliable such as monthly magazines – physics spoectrum , maths today etc . 
else u can take trishna foundation and askiitian foundation live are also awesome , 
ALL THE BEST ..