"Published in England by the Association of Photographers (AOP), Beyond the Lens is the essential guide to rights, ethics and business practice in professional photography.
Now in its third edition it has been likened to 'the bible for photographers and commissioners' and is used by colleges/universities as part of their courses and widely used by photographers and commissioners." (taken from AOP website)
Might be useful because...
This website has an 'appendix.pdf' that you can download with templates for writing estimates, model release forms, invoices, license agreements, and also invoices etc for photographic assistants.
Click here and go to 'standard forms' in the left-hand navigation!