The American Society of Media Photographers (ASMP) provides many online resources that can help you work with professional photographers. Working With An Assignment Photographer is a good place to start. ASMP also provides a guide to licensing photography.
Depending upon your location, the nature of your assignment and the method in which the photography is delivered, you may be charged California Sales Tax. This new guide to Sales Tax for California Photographers can help you understand when photography is taxed and when it is not.
The Advertising Photographers of America (APA) provides an online business manual for photographers that provides insights into the many considerations for hiring an assignment photographer.