Best reference books for science include NCERT Science Books, Science Textbook (Physics + Chemistry + Biology) by Lakhmir Singh and Manjit Kaur, Super Refresher Science for Class 9 by MBD Group, Physics Class 9 by Lakhmir Singh and Manjit Kaur, S Chand Publication, Science Chemistry for Class 9 by Modern ABC Plus, Super Simplified Science Biology Class IX by S. Dinesh Publication, etc.