Pricebooks can be imported in, from either the Pricebooks function, or within the Pricebook itself.

To Import the Pricebook file, open the spill menu at the end of the row and choose Import Pricebook.

Alternatively open the Pricebook to import into. Select More Options and choose Import Pricebook. 

Both options bring up the Import Pricebook window:

Select the rule to apply to the imported Pricebook. This will be populated automatically if a rule was selected on the Product Pricebook.

Either drag the Pricebook file or browse for it using the Browse File option. 

Enable File includes header row if the first row on the Pricebook file has headers. Choose Import to proceed and bring up the confirmation message:

Choose Yes to complete the Pricebook import. This adds all of the products in the Pricebook file to the on screen Pricebook: