You can still use this
http://nookworks.blogspot.com/2012/0...d-card-as.html
You'll need to find that Simple Touch Glowlight since the Glowlight Plus lacks the SD card required.
You wont be able to access the BN purchased books on your Plus without rooting it, which if you want to do then look over on the xda forums as I believe they've got it done. That said the last I was aware it wasn't a simple process.
For the method I suggested you'll pretty much need to know how to follow instructions, none are overly complex though I think you may need to use -gasp- the Terminal for OS X. I don't recall as I set up a Nook to do this years ago and still have the SD card floating around.
And unless BN decides to update the firmware for a line of devices they have literally ignored for years this wont break unlike the Nook Study/Nook for Mac/PC method.
eta- if your goal is Calibre managment you'll also probably want to investigate Alf and DRM which can be done in a google search. Otherwise you may be limited in terms of what you can do with the ebooks.