Nook GlowLight Plus folder destination should be NOOK/My Files
The new Nook GlowLight Plus (Fall 2015) doesn't seem to work exactly like previous models. It will see side loaded epubs in the NOOK folder (Calibre's default destination). However, the Keep Reading button that returns the user to the current book doesn't work unless the epub is located in NOOK/My Files. Barnes and Noble have updated their support pages to specify the My Files folder as the location for all user content.
Workaround: Once connected to Calibre, "Configure this device" from the Device menu, and change the first folder in the list from "NOOK" to "NOOK/My Files".