Many online stores provide relevant products in product pages or other pages to attract customer interests. Association rules based on customer purchases and personalized recommendations are most prominently used ways of providing relevant products. Though there have been many studies to apply tags of products as metadata of the products, there are few studies to investigate contents similarities between the products and the suggested products. Thus, this study collects books in purchase associations and their tags in Amazon.com and assesses the similarities between the books. We found out that the contents similarities based on tags are similar among business, literature, and computer networks. And the similarity is also similar among the relevant books that have different ranks.