This article takes you deeper inside GATE – it discusses in detail what are the the three different types of questions asked in GATE along with examples.
After you read thoroughly the above articles, now it is the time to actually start your preparation for GATE. First go through the GATE IN syllabus. Than attempt the previous years exam papers to find which are your strong/weak areas. Lastly buy the books to improve your weak areas.
Download Syllabus for GATE 2013 Instrumentation Engineering (IN) Paper:
This is available in the official GATE website.
Download Previous years GATE Instrumentation Engineering Exam papers:
While downloading these papers, please bear in mind that there was a major change in the pattern of GATE since from GATE 2009. Prior to GATE 2009, there were 85 questions for 150 marks. In GATE 2009, there were 60 questions for 100 marks. Since from GATE 2010, there are now 65 questions for 100 marks.
You can use also the book below for previous years GATE Instrumentation Engineering Exam solved papers:
GATE Instrumentation Engineering Exam Prep Books:

Not only for GATE, but also for any competitive exam (be it an entrance or a recruitment exam), you should buy book depending on your actual requirement – which is to strengthen your weak areas. As I told you previously, you should identify your strong/weak areas by attempting the previous years GATE exam papers.
GATE Coaching institute for Electronics Engineering:
So you need coaching for GATE? Well, I am sure with self-preparation you can crack GATE easily. However, if you need the help of coaching institutes for GATE IN paper, may be you can try with
this list given by inspirenignite.
Hope above compilation of GATE Instrumentation Engineering prep materials and study plan would help in your preparation for the exam. I would be on the look out for more resources and post the same here. So keep visiting here for GATE Instrumentation Engineering study materials!