One approach to Demand-Driven Access (DDA) is to make the entire Ebook Central catalog available to patrons, but then mediate all purchases (or STL) requests.

To do this:
- Go to your LibCentral site and sign in
- Under "Settings", select "Access Permissions"
- Either edit an existing Permission ID by clicking the desired one, or create a new one first by clicking "Add Permission"
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- On the "Access Permission Details" page, set "Visibility" to "Entire Catalog"
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- Click "Save Settings"
- Click the "DDA" tab
- On the DDA tab, set "Default Access Type" to "Mediated"
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- Click "Save Settings"

Then just be sure this Permission ID is assigned to any patrons you want to be able to view the entire catalog and request desired books.