It’s always best to move a level of abstraction upwards with this type of thing, that is I’d avoid Test books for Swift unless you really want to know the nuts and bolts in your first language. I’d go for this type of thing: https://www.amazon.co.uk/Test-Driven-Development-Addison-Wesley-Signature/dp/0321146530/ref=sr_1_1?dchild=1&keywords=tdd&qid=1588001042&sr=8-1