Best Book for Topicwise Previous Year Solved Papers with Solution?

The best book for Topicwisw Previous Year Solved Papers with solution is Made Easy Question Bank Book. This book covers questions on all the subjects coming in Gate Exam. From the index, one can select the topic of a  particular subject and practice the questions on it. Another advantage of this book is that answer key of topic is mentioned at the back. 
But the disadvantage of book  is : Explanation of correct answer is not given and only the correct answers are mentioned.
However, this book covers almost all the topics of each subject. I will suggest if you are buying this book, then make sure you will complete this book more than twice. After practicing the questions thrice or more you will find that your conceptual knowledge has increased plus you have practiced various types of questions on various topics. 

Apart from this, you can also refer to geeksforgeeks , an amazing website for studying gate subjects and practicing previous year questions. You will find Gate CS corner and Gate notes on geeksforgeeks.