Do I have to use Yuzu Apps to read my purchased/rented etextbooks?

Yes.

For all Yuzu Catalog digital content purchased through BNCollege websites or Yuzu.com you must use one of the Yuzu eReader Apps. 

eTextbooks are created by the same Publisher of the print version of a textbook.  The digital versions of the textbook are distributed with the requirement of DRM (Digital Rights Management) protections.  Each retailer selling eTextbooks is required to have an eReader App available to manage the etextbook license by providing access to the content.

eTextbooks are by contractual license from the publisher not available to download independent of the Yuzu Apps.