The acronym R.U.T.E stands for Rute Users Tutorial and Exposition. Rute is a recursive acronym like GNU - which stands for GNU is Not Unix. Rute is also pronounced exactly like "root" which is the Unix system administrator's login. So Rute is hence a book for system administrators.
In days-gone-by there were two kinds of books for teaching Unix: books meant specifically for those administrating Unix systems, and then books meant for those who were merely users of Unix systems who did not have administrative privileges for the machines they used.
Today, however, anyone who does anything complicated with a Linux system is also a "Unix System Administrator" by definition, although they may not have system administrator status in terms of their experience or position. Hence titling a Linux book a "System Administrator's handbook" is going to be lost on its intended audience.
A "root user" is a more accurate definition of the intended audience. Merely to use the desktop of a pre-installed system would not interest such a user. This is someone who is most likely going to do far more: like install, configure, and customize the system. To do all this, you need to log in as "root" fairly often.
You can find out what book a person needs by asking the question, "Do you want to be a Muggle or a Wizard?"
(1) If they answer "Wizard", then you give them Rute.
(2) If they answer "Muggle", then you give them "Linux for Dummies."
(3) If they answer "What's a Muggle?", then you give them "Harry Potter".