I wish to create a Page library that has some shared items that everyone can see, and some items that just some users can see. What might be the best way to implement this?
You will have to "break" the libraries permissions and then assign permissions to each individual item.
This isn't advised though as there's a potential for performance issues.
Microsoft produce a Best practices for using fine-grained permissions (SharePoint Products and Technologies) guide which will be worth reading.