How do I add or remove pages in my book?

Not page by page. Your book is laid out automatically, so the page count is a result of what is in it rather than something you set directly.

To make the book shorter: narrow the period under Select the time period to be printed, untick posts in Edit content, or choose a smaller Picture Size.

To make it longer: widen the period, add albums in Choose Content, or choose Big or Very big under Picture Size.

Two limits apply. A book must reach 20 pages to be printable, and it cannot exceed 500 pages. If you go outside either, the editor tells you and suggests what to change.