Catchy. I like it. It's kinda cute. For comparison, BookRat.com has been registered since 1994 and nothing has been done on it.
Since it's a creative, catchy name, it has potential. That being said, there is a trademark on MOUSE BOOK.
Also, the following domains are registered:
bookmouse.club
bookmouse.net
bookmouse.org
bookmouser.com
bookmousey.com
bookmousepublishing.com
Potential: $xxx - low $x,xxx