The Guestbook, a hotel rewards site that is making its debut today, gives travelers 5 percent cash back for their stays. Users, who sign up with their email addresses, can make a reservation either through the Guestbook site or on the hotel’s direct site and redeem their rewards through PayPal or gift cards to more than a dozen retailers such as iTunes and Amazon. The site has partnered with 300 independent and boutique hotels around the world for the debut, including the Hudson in New York and St Martins Lane in London and has a promotion through the end of November in which all new enrollees receive $25 toward their first booking.