Moon and Back Bookstore in West Medford is celebrating Independent Bookstore Day by taking part in the second annual North of Boston Book Trail Passport Challenge. The event encourages book lovers to visit as many local indie bookstores as they can on Saturday, April 26, and Sunday, April 27. The more bookstores you visit, the bigger the prizes you could win. Moon and Back, a kids' bookstore that opened in February, is one of 19 participating bookstores. Each store will…