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 Storybird Writing Club Membership. This places your books in a separate moderation queue that's reviewed before any other books. Your books clear the queue within 24-48 hours and are available for everyone to see.