98% of the questions are always asked from NCERT. The rest 2% is asked from other books. Topics like Frog and Earthworm though have not been in NCERT have been asked in the previous years. Now it is too late to buy any book to cover those subjects. So stick to NCERT and then practise atleast 1 mock test a day. That way you will have prepared atleast 15 mock tests until September 15th and you will also be prepared for our of NCERT books questions.