For preparing GATE you should refer standard books for each and every subject.

Along with these there are some other books which contain many unsolved problem which will check your concepts and increase your problem solving ability.

Thermodynamics

• Engineering Thermodynamics by PK Nag
• Introduction to Chemical Engineering Thermodynamics by JM Smith, HC Van Ness and MM Abott

Fluid Mechanics

• Unit Operations of Chemical Engineering by Warren L. McCabe and Julian C. Smith

Mechanical Operation

• Unit Operations of Chemical Engineering by Warren L. McCabe and Julian C. Smith

Heat Transfer

• Heat Transfer by Cengel

Chemical Technology

• Outlines of Chemical Technology by Dryden

Plant Design and Economics

• Plant Design and Economics for Chemical Engineers by Timmerhaus and Peter

Instrumentation and Process Control

• Process Systems Analysis and Control by Donald R Coughanowr

Chemical Reaction Engineering

• Chemical Reaction Engineering by Octave Levenspiel
• Elements of Chemical Reaction Engineering by H Scott Fogler

Mass Transfer

• Principles of Mass Transfer and Separation Processes by BK Dutta

Process Calculation

• Basic Priciples and Calculations in Chemical Engineering by Himmelblau

Mathematics

Higher Engineering Mathematics by BS Grewal

All the above books will help you in completing syllabus.Follow these books according to GATE syllabus.

In addition to the above books, for practising wide variety of questions kindly follow below mention books which will enhance your practice and accuracy

• Chemical Engineering for GATE by Ram Prasad
• An insight to Chemical Enginnering by M Subbu

30–40 days prior to GATE exam start solving previous year Question paper because you will get to know what type of concepts are important in which subject.

At last take test series of some 1–2 coachings which you will find helpful in time management.