Hi,

ISI entrances test your basic mathematical and problem solving ability. You need to give special focus to Calculus, Geometry, Inequalities, Number theory etc. Some books by standard foreign authors will aid you in preparation for these topics. The authors include IA Maron,  David Burton, Arthur Engel etc. For interview, they generally ask you to solve few tricky mathematics questions itself. Just be confident and answer without fear.

All the best.