Hii

If you would specify which subject are you going to appear for in GATE exam then i would be able to help you witht he best books ffor you preparations. As such there is nothing called best book, you just have to make sure when ever you get a book for yourself it has the following qualities - 

• Simple in language and easy to understand
• Has illustrations and diagrams for explanations
• Is not very complicated rather must be crips and concise
• Must have accurate details and clarity must be given in each topic
• must be based on the syllabus itself.
• and must include sample questions and practice questions
  

SO if you could mention the subject, i would be able to help you with the best of my knowledge.
You can refer to following link for more details - https://engineering.careers360.com/articles/what-are-best-books-for-gate-preparation

Thanks!!