You can rename book in the Control Center. Go to "Control Center", "Books", select your book and then in top right corner you will see button "Rename".
It was possible to delete person and book but it was temporarily removed in version 1.5.4 due to some security reasons. New support for deletion of users/books/chapters will come soon in version 1.6.
I think I figured out the problem on systemswiki.booktype.pro
Being an absolute bumbling newbie I managed to created a user systemswiki though genebellinger is the owner with Control Center access. And it was the systemswiki user that created the book, so systemswiki can't get rid of it because they don't have admin access and I can't get rid of it because it's not my book. I guess I'll just wait for v1.6, which is soon I hope.