Some miracles make the book great. The change in the reader's life comes first.
But this is almost impossible. Every book changes the reader's life. Little changes make us who we are.
But for the reader to say that this book has changed my life, there must be a real change.
I hope you'il write a book like this one day.
Another thing that makes a book perfect is understanding.
The human brain makes rapid transitions between subjects and is not the best focusing species of the universe. If you write a complex book, it will probably not be read.
Font selection is the shortest way to make a book read and understandable.
Serif fonts make it easier to read.
I also mentioned above how many font sizes you should have.
Of course, all this information applies to printed publications. If you will only publish your book on the Internet, please search the Internet.