Besides the main content of a book, the parts of a book itself can provide information useful for research. The main parts of a book include:
Table of Contents: An outline or breakdown of the different chapters of a book.
Bibliography: The list of works used in the research of the book; useful for finding more informaiton on a topic.
Index: The list of key words, often including people and place names, used in the book and the page numbers for where those words appear; typically the last section of a book. Useful for knowing whether a specific person or term is referenced within the book.
Other possible sections include:
Appendix: The part or parts of a book that provide additional information to the main text; usually found near the back of the book.
Glossary: The section that defines key terms used in the book; usually found near the back of the book.
Another type of secondary source is a scholarly journal article. These are much shorter than books, but also much more specific. You won't find a complete biography of Marco Polo in a scholarly journal article, but rather an analysis of a specific part of his life or work. Scholarly journal articles often have a thesis and a specific point of view, which is something to keep in mind as you read and take notes.
Here are two databases that contain scholarly journal articles:
You can also find some journal articles and secondary sources on websites.
However, not all websites are trustworthy and worthy of citation in your academic work. Be sure to consider who is responsible for a website, when and where it was created and updated, and for what purpose. Always ask your teacher or a librarian to vet a website before citing it in your paper.
Below are some websites with secondary source-type information that you can use in your research: