You cast Secluded Grimoire, then travel to the Outer Planes. Is your book presumably inaccessible?
Is the Night Hag presumably a creature that leaves for the Material Plane? Because it's ability to go ethereal shouldn't work in the Outer Planes where there isn't an Ethereal.
Why would it be inaccessible? Is there something I'm forgetting about the outer planes and ethereal planes?
Edit: Looking into it, it looks like the ethereal plane is only in a small portion of the Outer Sphere. So yeah, I guess you wouldn't be able to access it.