Try searching for the Psychological First Aid manual, I believe put out by a national PTSD organization. It's a quick easily learned approach to crisis intervention that has been published in a manual intended for use by all types of first responders who may end up being crisis workers.
Also currently I'm taking a crisis intervention class this semester and this book has been pretty thorough so far and you could find a used text for pretty cheap https://www.amazon.com/Cengage-Advantage-Books-Intervention-Strategies/dp/1111770611
Edit: here's a link to the psychological first aid manual https://www.ptsd.va.gov/professional/manuals/psych-first-aid.asp