@danielsantos On the search result page, there should be an Add to bookshelf… section where you can add the book to one of your shelves. But only if you're signed in to your Micro.blog account.
Sometimes when you follow links, they open in in-app browsers or a browser where you're not signed in. When that happens, you must sign in or switch to a browser where you're already signed in. Also, see this conversation. Ping @manton.