Reticent
Definition: holding one’s tongue; uncommunicative
This is sometimes also used as a synonym for “reluctant.”
Why I Looked It Up
I didn’t note the book, but I have this in my notes:
There was something calming in the reticence of all those books, their willingness to wait years, decades even, for the right reader to come along and pull them from their appointed slots.
Postscript
Added on
In an article about King Charles’s cancer diagnosis:
The disclosure was a break with the family’s tradition of being reticent about making health issues public.