The difficulty of a math book is subjective and can vary based on individual preferences, learning styles, and prior knowledge. However, some math books are known for presenting challenging problems and in-depth concepts suitable for students seeking additional enrichment or those preparing for competitive exams. Keep in mind that the level of difficulty perceived by students can vary, and what may be challenging for one student might be manageable for another.

One of the books that is considered challenging and is often used for enrichment or preparation for competitive exams is:

"Challenge and Thrill of Pre-College Mathematics" by V. Krishnamurthy, C.R. Pranesachar, K.N. Ranganathan, B.J. Venkatachala:

• This book is designed to stimulate interest and encourage creative problem-solving skills in mathematics. It covers a wide range of topics and includes challenging problems that go beyond the standard school curriculum. It is suitable for students looking for a deeper understanding of mathematics.

It's important to note that while challenging math books can be beneficial for enhancing problem-solving skills and critical thinking, they are not always recommended for every student. It's essential to ensure that the difficulty level aligns with the student's current understanding and readiness. Students and educators should use discretion when selecting books and consider factors such as the student's goals, motivation, and enjoyment of the subject.

Before choosing a book, it's advisable to consult with teachers, mentors, or educators who can provide guidance based on the student's current proficiency in mathematics. Additionally, understanding the curriculum requirements and focusing on mastering core concepts is crucial for success in academic pursuits.