This tweet warms my librarian heart. It's always a good idea to keep emergency books around. Hurricane season is upon us here in Texas, and I'm always careful to keep a supply of print books to enjoy if the power goes out for a few days.
(More seriously, I have expressed to my library director that, as a matter of collection development policy, it should be possible to rebuild human civilization from the print collection of any library. This saved some unread volumes from disposal.)
I exhausted the libraries of my school, the local branch and the regional library before I entered high school, and had to order then from the main branch of the Chicago Public Library. Now in my 70's I still find time to read a book per day and though authors are more numerous and prolific, each year it gets harder to find new ones worth reading.
I would NEVER discourage a 9 year old, who loves reading so much that she thinks about what she'd do in an emergency, from taking as many books at a time as permitted by the library. The only dangers are that she may run out of books to borrow that she wants to read, or she'll learn to read a lot better than her peers. That second danger will give her a big boost as she progresses through school.
I WOULD get her a tablet and set her up with online access to a free lending collection ASAP.