The reason page numbers do not display on O'Reilly ebooks is because the text is free-flowing and will shift to fit the page size, so the text on any given ebook page is dynamic.
If you need to reference an ebook in a specific style (Harvard, Vancouver etc.) the Library has a Citing and referencing guide
Learning Development Service also have Guide to Referencing
Consult your module handbook or check with your lecturer to confirm the required referencing style and format for ebooks.