Alternating books and magazines on a shelf would not be considered cross-merchandising, because books and magazines are not complementary products. Customers who are looking for a book are not likely to be interested in a magazine, and vice versa.
However, if a store were to place books and magazines next to each other in order to encourage customers to purchase both, this would be considered cross-merchandising. For example, a store might place a book about gardening next to a magazine about gardening, or a book about cooking next to a magazine about cooking.