The pie slices are not the book’s rating! It simply tells the reader, at a glance, what kind of book this is.
There are hundreds and hundreds of how-to books for writers, all of them giving you different advice. The instruction can be put into three broad categories: craft (how-to), business (selling your work) and inspiration.
If a book were a key lime pie with eight slices, how many of those slices would be craft, how many would be business and how many would be inspiration?
In addition to slices, a book also gets a traditional star rating.