By default, users will only have access to the "Default" pricebook of the Core they are attached to. If a user attempts to change their pricebook, they will see an empty list like this:
To enable access to other pricebooks, a Regional Admin must create a pricebook user group and add the user to the group.
As a Regional Admin:
Go to Admin Tools
Open Price Book
Select Pricebook User Groups tab
Create a Group Name (Typically the same name as the pricebook) and select the Pricebook.
Add all users that are allowed to use the pricebook and click save
The user(s) will have immediate access to use the Pricebook drop-down from the Tools page.