Where are the files?

In your books folder (Settings, Books folder), inside "_Activity Studio", in the book's own folder (named with a code, not the title), in "exports": the interior PDF and the page images zip, and the printable edition in "exports/Printable". The files are named after the book's title, for example "Title - Interior.pdf", "Title - Printable (US Letter).pdf", or "Title - Puzzles (A4).pdf" and "Title - Solutions (A4).pdf" when the solutions are apart.