Dear Aspirant,

     I appreciate your interest towards ISRO and I'm here to help you regarding your query!

     Generally, the syllabus for this is almost similar to the GATE examination syllabus and it is enough if you prepare with any good GATE books. There will be more weightage to communication in ECE than ant other topics. ISRO paper will majorly ask lot of questions feom theory part and hence, you can omit problematic questions. The most intelligent way to crack this examination is to practice lots and lots of previous years examination papers thoroughly. You can also refer some best books such as,

• Principles of Electromagnetics, by Mathew N.O. Sadiku.
• Principles of Communication Systems, by  Goutam Saha, Herbert Taub , Donald Schilling.
• Semiconductor Physics and Devices by Donald A. Neaman, Dhrubes Biswas.
• Fundamentals of Electric Circuits, by Charles K. Alexander, Matthew N. O. Sadiku.
• Microelectronic Circuits: Theory and Applications, by Adel S. Sedra, Kenneth C. Smith.
• Digital Logic and Computer Design by M. Morris Mano.
• Signals & Systems by Allan V. Oppenheim, Alan S.Willsky, S.Hamid Nawab.
• Microwave Engineering by David Pozar.

Hope this helps !!

All the very best wishes for your future endeavours !!