As you want to get the list of some of the best reference books for the Graduate Aptitude Test in Engineering Instrumentation Engineering exam so here is the information of the same for you:

Books for Engineering Mathematics
• Textbook of Engineering Mathematics by Dutta Debashis
• Advanced Engineering Mathematics by Kreyzig E
• Advanced Engineering Mathematics by Potter M.C, Goldberg J.L and Aboufadel E.F
• Higher Engineering Mathematics by Ramana B.V
• Higher Engineering Mathematics by Grewal B S
• Advanced Modern Engineering Mathematics by James G

Books for Industrial Instrumentation
• Instrumentation Reference book by B.E. Noltingk
• Process/Industrial Instruments and control handbook by C.M. Douglas
• Principle of Industrial Measurement by D. Patranabis,

Books for Circuits and Measurement Systems
• Measurement Systems: Application and design by E.O. Doebin

Books for Analog Electronics
• Microelectronic Circuits by Sedra and Smith
• Electronic Circuits by Nilsson and Riedel
• Microwave Engineering by Pozar
• Design with Operational Amplifiers by Franco
• CMOS Analog Circuit Design by Allen and Holberg
• Circuit Design, Layout, and Simulation
• Analog Integrated Circuit Design (Johns & Martin)
• Analog Design for CMOS VLSI Systems – Franco Maloberti

Books for Digital Electronics
• Digital Principles & Application, by Leach & Malvino
• Fundamental of Digital Circuits by A. Anand Kumar
• Digital Fundamental, by Floyd & Jain
• Digital Electronics by James W. Bignell & Robert Donovan
• Digital Integrated Electronics by H. Taub & D. Shilling
• Fundamental of logic Design by Charles H. Roth
• Digital Logic Design by Morries Mano

Books for Signals, Systems and Communications
• Voice and data communications handbook by Bates, Regis J
• Signal processing handbook by Chen, C. H.
• Handbook of digital signal processing by Elliott, Douglas
• Reference Manual for telecommunications engineering by Freeman, Roger L

Books for Electrical and Electronic Measurements
• Solutions of Network Analysis by GK Publications
• Network Analysis by Soni and Gupta
• Electronic Devices Circuits by Millman and Halkias
• Switching and Finite Automatic Theory by Kohavi

Books for Control Systems and Process Control
• Control Systems Engineering by Madan Gopal and I.J. Nagrath
• Technology & Engineering by Butterworth-Heinemann
• Control Engineering Theory & Practice by Bandyopadhyaya
• Modeling & Control of dynamic system by Macia & Thaler

Books for Biomedical Instrumentation
• Biomedical Instrumentation: Technology and Applications by R Khandpur
• Biomedical Instrumentation and measurement by Leslie Cromwell Fred.J. Weibell
• Medical Electronics by Kumara doss