Ok, I've figured this out and thought I'd post to let anyone who finds this via search how I fixed it....
After a bit of tinkering around I found out that the Nook's author sort method is little bit backwards....
The Nook doesn't use the "author_sort" metadata field, it uses the very last word of the "authors" field, then any preceding text.
Let's for the sake of example presume a book has a completely different "author_sort" to the meta-data contained in "authors".
author_sort:
Eddings, David
authors:
George R R Martin
The Nook, instead of sorting by author_sort sorts by
Martin George RR.
To fix this I created the following custom columns;
read - Yes/No
author_prefix - {#read:contains(Yes,zzz,)||} -
Returns zzz on books I've marked as read.
unread_sort - {#author_prefix:||}{author_sort} -
would return zzzMartin, George R R on book's I've marked read.
I then have the following plugboard setup;
[{#unread_sort:swap_around_comma()}] -> authors -
Would return George R R zzzMartin on books I've read.
This forces all the books I've already read to the bottom of the list of my Nook which makes finding my next book to read much easier.
Hope this helps someone.
Thanks,
Ryan.