Amazon’s Prime Book Box subscription service no longer requires an invitation and is now available to all Prime members in the US. The service costs $22.99 per delivery and includes 4 board books or 2 hard cover books, depending on which age range you select. The books are hand-curated for kids and can be delivered every 1, 2, or 3 months. Amazon says the subscription service will save customers up to 35% off the list price of the included books.