I have supplied you 2 example of booking letters you should check out. There is a comprehensive Entertainment Agencies contract sample and a very basic letter used by acts/bands/venues to confirm bookings, both of which can be adapted to your requirements.
SAMPLE BOOKING LETTER
Donít be put off by its terms and conditions as venues, agents and performers alike tend to work things out between them. For example, a club or pub booking a rock band would not expect them to wear tux and tails! The same follows for any problems that may arise, you should always endeavor to reach a compromise or alternative agreement, especially if you want to perform at the venue again. In the first instance contact the agent or manager who issued the contract who will try to negotiate on your behalf.