Maybe put the following text in a highlight bar on top of the page: "Already have a ticket book with us? Login and manage your tickets!", with the "login" word made into a link that pops up the actual login form. Visible enough, yet not intimidating. You may change the words "ticket book" according to what your marketing decides to call it; the word "account" is pretty common, yet I feel it may be too generic for your specific case and that's why I'm calling it a ticket book.