Yes, the Rare Books Department is a place.

Erasmus shakes his head at me and tells me I've taken terrible liberties with his beloved Rare Books Department. (I have and I'll try to be sorry about that at some point.) It's only fair that I set a few things straight. So to speak.

The Rare Books Department is an actual place in the main branch of the Free Library of Philadelphia, located on Vine Street. Also called the Parkway Central branch, since its right there, visible when you're on the Parkway. Rare Books is on the third floor and it really does have a glass-door entrance with gold lettering and it really does house the stuffed and mounted body of Grip, Charles Dicken's talking raven, in a glass case. As an aside, Grip is said to be the inspiration for the raven in Edgar Allen Poe's poem. Maybe? But the neon raven in Offbeat Crimes is called Edgar for the poet, so there's some kind of completed circle there.

While the staff in Rare Books takes great care for their collection, I've probably understated their public outreach efforts in the stories. The department is there, as its web page states, to serve the people of Philadelphia, as well as the students, scholars, and researchers who use its collections. Daily tours, exhibitions, and classes help to open the department’s collections to a wider public.

You can visit for tours Monday through Saturday, starting at 11am. You can make appointments for research in the reading room Monday through Friday. You can see the Elkins room, which was given to the library by William Elkins, who not only donated his books, but his actual library.

And, yes, they really do have an outstanding collection of Beatrix Potter illustrations and sketches with, I'm sure, an expert in her work whom I so ungraciously shoved aside to give the job to Erasmus. I very much doubt that they have locked cabinets of spell books, but how would I know? Maybe it's in the back somewhere.

They do plenty of special exhibitions as well. The current one as of this writing is "Big and Small, Books For All," of extraordinary books selected for their extreme sizes and unusual shapes, which runs until March 2018.
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It's such an unusual and wonderful place, I couldn't possibly have made it up. For more information, please see their web page and go visit when you can.
https://libwww.freelibrary.org/programs/rarebooks