All answers are thorough on this, just wanted to put in my two cents that Prime would never be worth it for the books alone, that is a minimal and side deal. Most of the prime books will be offered the following month for free if you have unlimited anyway, or else they stay considerably cheap and go on sale often.
Our household uses Prime Video quite a bit but mainly by adding on extra channels to supplement, have to have that BritBox for our Poirot, you know
We use the shipping as handy but it's not that often anymore, mainly around holidays.
I have my account without prime since I use my parents for the Prime purposes. When I ordered something recently on my account, the time difference to receive it with free shipping versus Prime free shipping was about the same anyway. I must have warehouses near me with that particular order. It was estimated two weeks but took 2 days.