Hi Aspirant

Buying books for separate subject might be hetic for you because they might distract you from the exact syllabus of GATE.You need to be very much concentrated to  the syllabus only.

You can buy materials of MADE EASY,ACE Academy,The GATE Academy,Gate Academy .These are the best institutes for gate preparation and their and test seris are also up to the mark.These materials will be really helpful.Apart from materials you ca join offline or online course whichever is possible .

All the best.