5 stars ought to be enough. I usually either like or dislike a book, simple as that, so I'd probably give everything either 5 stars or 1-2 stars. Speaking for myself, there are no cases where I'd want to give something 9 stars or 6 stars out of 10.

Here's an example of the book cover issue: https://ibb.co/DQLC584

In the pic, out of five recommendations, one is correct, two are wrong, and two have no covers available.