The subject matter that a book or article treats is the field or area that it touches on. For example, the subject matter of Dickens's novel A Tale of Two Cities is the French Revolution and its effect on people. The main point of a book is the principle idea the work is trying to convey. For example, the main point of A Tale of Two Cities is the nobility of sacrificing oneself to save others, particularly during difficult times such as the French Revolution.
Of course, the subject matter and the main idea can be similar and overlap, but they are not always the same. It might be useful to think of the subject matter as where you would find that book in the library. For example, a book about fish would be found in the animal section of the library. However, different books about fish can have different main ideas. For example, one book might discuss the study of fish, another might be about extinct fish and why they became extinct, and another might be about how to fish. These differences would be related to the main idea of each book.