The World Baseball Softball Confederation (WBSC) has published a stand-alone Official Slowpitch Rulebook for the first time, valid from 2018-21.
Previously, the WBSC and ISF had published combined Official Rulebooks, with the rules for fastpitch, slowpitch and modified pitch formats jumbled up together. The decision to create a Rulebook solely for slowpitch came about after many years of lobbying by the British Softball Federation along with other countries.
You can view or download the WBSC Slowpitch Rulebook here: http://www.britishsoftball.org/uploads/_documents/Files/WBSC%20Slowpitch%20Rulebook%202018-21.pdf.