Is it possible to hide a Library from the left navigation menu?
No, it is not possible to hide a Library from the left navigation menu.
Can I create a Library that's only visible to Internal Users?
Yes, but you must submit a request to Channeltivity Support and they will create a Custom Permissions Resource for the Library. You'll then be able to give your Internal Groups access to the Library by going to Admin > Groups > select the checkboxes next to the appropriate Internal Groups > Actions > Add Internal Resource > select the new Library from the dropdown menu. When adding content to the new Library, assign the content to the appropriate Internal Groups.
Add Internal Resource Window