You say in your book that the final “s” in the words “is” and “has” is pronounced as /z/. Then what if when “it is” and “it has” are contracted to “it’s”, does the “s” remain /z/ sound?
When “it is” is contracted to “it’s” the S sounds like /s/, not /z/. This is because it comes after the “t”, which is a voiceless consonant. (Rule #1 on page 59 of my book, “Mastering the American Accent.”) Good observation Alex. Make sure you memorize that list (on page 60) of common words that end in /s/ which is pronounced as a /z/. I regularly have students that make this mistake. Even today I was correcting my Italian student who was saying “wasss” instead of “wazzz.”
I wish it were simpler. In most other languages these kinds of changes don’t occur.