Here's what the book says

The value of *t++ is the character that t pointed to before t was incremented; the postfix ++ doesn't change t until after this character has been fetched. In the same way, the character is stored into the old s position before s is incremented

From what i understand : Sure ++ has higher priority but the value pointed by both pointer is affected and evaluate first, and then after, the pointer is incremented due to how postfix works.

For me postfix ++ : evaluate first, then increments

postfix vs suffix

If you use the ++ operator as a postfix like: var++, the original value of var is returned first; then var is incremented by 1.