What's the little hook for on the side of the Hotel phone?

[Greg G.]

Here:

[bingster]

That's where you hang your telephone directory.  Lots of areas had telephone directories that were half the size of the larger ones (they were more like a thin booklet), and those usually had a little hole in the corner with a string loop to hang them from this hook.

[Greg G.]

That crossed my mind, but I was picturing today's big fat phone books, not the skinny one's of yesteryear.

[bingster]

The way these phones are built, they'd probably hold one of our modern monster phone books, too.