As you are appearing for ISAT exam so I am providing the list of reference books that will help you in preparation.

Physics
1. H C Verma
2. Arihant
3. Pradeep Fundamental
4. I E Irodov

Chemistry
1. P Bahadur
2. GRB Publication
3. TMH for Chemistry
4. R S Mukherjee
5. NCERT
6. Peter Sykes’ Reaction Mechanism for Organic Chemistry

Mathematics
1. M L Khanna
2. R D Sharma
3. Arihant Prakashan Books for Maths
4. Das Gupta
5. S L Loney (Trignometry)

I hope this list will help you in selection books for preparation.