You should select a good text books which is easy for you to understand as well as covers the entire spectra of Gate syllabus. And you should choose a good question practice book which is oftenly said to be refernce book. So the basic thing is there are very less books which have good theory as well as good practice problems. For that we have to split the book choices for our own benefit.

I hope my answer helps you. All the very best for your future endeavors!