Your work autosaves a few seconds after every change — watch for the Saved status in the top bar. The Save button is there for reassurance: it saves immediately instead of waiting those few seconds.
If the status ever says it could not save, don't close the tab — check your connection and click Save again.
Preview flips through the finished book exactly as it will print, with hidden pages left out.
Tap Help at the bottom right of any page and tell us what is happening. Your message comes straight to us and we usually reply within a few hours.