You have a few options for citing sources in your Pressbook. You can create a References section at the end of each chapter, create a list of references in the Back Matter of your book, or you can use the built-in footnotes tool.

You can add a footnote or endnote as follows in either the text editor OR the visual editor:

Call me Ishmael.[footnote]This is one of the most famous opening lines in the English language.[/footnote] Some years ago—never mind how long precisely...

This will automatically create a footnote (or endnote) with the text inside the [] "shortcode".

One nice thing about footnotes is that they can be configured in PDF exports. You can select to have footnotes appear as Footnotes (displayed at the bottom of the page) or Chapter endnotes (displayed at the end of the chapter).

More information about footnotes and endnotes can be found in Pressbooks User Guide: Footnotes and (Chapter) Endnotes.