OK, my major difficulty was trying to place the formula in the book list template itself - checking one of the 'related' threads reveals that the book list template does not evaluate expressions.
The solution is to create a custom column that evaluates to the desired value and putting the name of the column in the book list template:
#status column contains template
Code:
{tags:in_list($,', ',Completed,Complete,Incomplete)}
book list template contains
This appears to be generating correct results, although I see the occasional single quote mark returned instead of Complete or Incomplete. No idea what's up with that, gonna stop beating my head against the table for tonight.