Why is the table of contents in the abstract placed at the beginning of the document, and in many books at the end?

Because when reading an abstract, you first need to present its structure, that is, place the table of contents at the beginning. This is not required in books, if the reader needs to find something, he will turn to the end to its content.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.