All public stories on Storybird are moderated for safety and age-appropriateness.
Books are moderated continuously. However, given the enormous volume of submissions, it takes a while for a book to clear the moderation queue.
Free moderation can take a few weeks (depending on overall submitted story volume the wait can fluctuate).
You can speed things up by upgrading to Upgraded Membership. This places your books into the Express Moderation queue that's reviewed before any other books. Your books clear the queue within 24-48 hours and (if approved) are available for everyone to see.
You can also save up some Crowns and redeem them get one of your Picture books or Longform chapters into the Express Moderation queue.