# API Reference

- [Concepts](/legacy/legacy-experience/books/custom-books/api-reference/concepts.md): Understand concepts and how they are utilized in procedures.
- [Connections](/legacy/legacy-experience/books/custom-books/api-reference/connections.md): Learn how to connect with third-party tools and services in your custom book project.
- [Docstrings](/legacy/legacy-experience/books/custom-books/api-reference/docstrings.md): Understand how to format and write docstrings to ensure your Book is well-documented.
- [Decorators](/legacy/legacy-experience/books/custom-books/api-reference/decorators.md)
- [@book](/legacy/legacy-experience/books/custom-books/api-reference/decorators/book-decorator.md)
- [@concept](/legacy/legacy-experience/books/custom-books/api-reference/decorators/concept-decorator.md)
- [@config](/legacy/legacy-experience/books/custom-books/api-reference/decorators/config-decorator.md)
- [@connect](/legacy/legacy-experience/books/custom-books/api-reference/decorators/connect-decorator.md)
- [@oauth](/legacy/legacy-experience/books/custom-books/api-reference/decorators/oauth-decorator.md)
- [@oauthtoken](/legacy/legacy-experience/books/custom-books/api-reference/decorators/oauthtoken-decorator.md)
- [@procedure](/legacy/legacy-experience/books/custom-books/api-reference/decorators/procedure-decorator.md)
- [Enums](/legacy/legacy-experience/books/custom-books/api-reference/enums.md)
- [FilterBinaryOperator](/legacy/legacy-experience/books/custom-books/api-reference/enums/filterbinaryoperator-enum.md)
- [FilterUnaryOperator](/legacy/legacy-experience/books/custom-books/api-reference/enums/filterunaryoperator-enum.md)
- [Filter Expressions](/legacy/legacy-experience/books/custom-books/api-reference/enums/filter-expressions.md): Learn about implementing filter expressions in your custom book project.
- [Noun Phrases](/legacy/legacy-experience/books/custom-books/api-reference/noun-phrases.md): Learn about the concept of a noun phrase.
- [Procedures](/legacy/legacy-experience/books/custom-books/api-reference/procedures.md): Learn how to implement a procedure in a custom book.
- [Questions](/legacy/legacy-experience/books/custom-books/api-reference/questions.md): Learn how to implement questions in a custom book.