Prime Book Box is now readily available to most Amazon Prime members. (Amazon Photo) Amazon on Tuesday opened its own Prime Book Box subscription service to most U.S. Prime members. Originally announced in May, the application provides a box of novels each 1, 2 or 3 months for $22.99 per month. Children 2 and younger have four board books per box, and kids 3-12 have two hardcover books. Amazon says customers can save as much as 35 percent off list price. Subscribers can swap books from a list of choice selections before the box ships. Amazon isn’t the first to try this. For example, a service named Kids BookCase conducts $10/month, and Reading Bug Box starts at $18/month to $45/month depending on the amount of books included. That is Amazon’s latest attempt to bolster its Prime plan, which has over 100 million members. Amazon offers other subscription boxes, even from STEM toys to handmade cured meats.