Yes, you can create a private catalog by selecting 'Private' status in the catalog details. The books will be uploaded to your website, but will not be viewable/searchable. The books can only be viewed with a direct link to the catalog.
If you do not see 'Private' as a status option, please email email@example.com.
If you would like to add additional items to your online catalog without uploading a new file, you can add items by ID.
Click on 'Manage Inventory' under the 'Inventory' heading in the Control Panel menu. Click the 'Bulk Edit Catalog' tab. Then, select the catalog you wish to update, input the item IDs, and click 'Create Online Catalog.'