I see @MaDill’s point and how this would further AntennaPod’s evolution towards being both a podcast-catcher and an audiobook player (assuming that is its strategy).
However I also see that the programming required to create the schema as described in the first note may not be trivial (as shown by @ByteHamster’s note), the processing required when in operation could be convoluted and the chance of requiring maintenance, such as with new versions of Android, as being potentially high. Also the workaround of creating a subscription for each sub-folder takes less than a minute, and later removing it even less, for which @Matth78’s suggestion has merit. Nevertheless, I take more time than the workaround to pick a dead-tree book off my stack of new ones and after reading it putting it away on my permanent shelves.
I therefore feel that this feature, like the 6 (so far) I have added myself is a low priority “nice to have”. I also feel and encourage that instead the efforts of the volunteer developers be concentrated on issues of much higher priority (I am not qualified myself to add them as features), such as any that can further improve the usability of AntennaPod for blind and visually impaired users.
(Yes - in my former life I was once responsible for managing the development and maintenance of major software systems - such as government accounting and payroll - also explains why I write such long and wordy notes - Sorry!)