Book

Robert A Curedale has written in 2013 a book called "Service Design: 250 essential methods" which offers one of the biggest overviews of Service Design methods I've seen so far.

Websites

The website Service Design Tools is one of the references for finding a list of Service Design-related methods. It also has a pretty interesting section called Enhanced Tools. There you'll find methods that avoid some of the weaknesses of old school Service Design work.
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There is also the This is Service Design Doing Method Library with 50+ methods with each a pretty good description. If you like those, you should check out the book This is Service Doing.
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The design team behind Capital One has created an illustrated poster of Service Design Tools & Methods.

If you want to go beyond Service Design, I have a list of "Methods Libraries" that you might want to check. These are websites that list design, innovation and other methods that can be helpful.