Working Effectively with Legacy Code by Michael Feathers is a good resource for how to introduce testing code into an existing system: https://www.amazon.com/W

Working Effectively with Legacy Code by Michael Feathers is a good resource for how to introduce testing code into an existing system: https://www.amazon.com/Working-Effectively-Legacy-Michael-Feathers/dp/0131177052/ref=sr_1_1?dchild=1&keywords=working+with+legacy+code&qid=1616769795&sr=8-1 https://www.amazon.com/Working-Effectively-Legacy-Michael-Fe...