Reference book serves as the purpose of understanding topics if it is not given properly in the main books or if you want to solve more questions on a particular topics you can practice and get good grip on certain topics and clear doubts or can clear when certain different questions arise from a topic you can use reference book to read more about that topic and much. But don't follow two books at a time follow one book and try to clear difficult topics from reference book.Thank you

Hello Aakash, competetive exams like NEET requires students to answer plenty multiple choice questions of varying difficulty levels in a short amount of time. To be able to score well in NEET, you need prior preparations and enough practice of answering NEET pattern questions. So get your Concepts cleared through textbooks and doubts cleared by your teachers or books and reference books are mostly used for Practicing ample questions before the exam. Also practice previous year papers and sample papers, a good reference book would contain enough concept building text and problems along with sample sets or previous year's solved questions.