The book with more mass feels heavier in your hand because mass is a measure of the amount of matter in an object. The more mass an object has, the more matter is packed into it, making it denser and thus heavier. When you hold a book with more mass, you are essentially holding more matter in your hand, resulting in a greater force of gravity acting on it and making it feel heavier. Additionally, the increased mass also means that more force is required to support the weight of the book, adding to the sensation of heaviness.
