The code should be something like this:
Code:
program:
if $#complete == '100' then
if $#read == '' then "Read in paperback or on another eReader."
else
format_date(raw_field('#read'), 'yyyy')
fi
fi
Where if custom column "complete" is 100, you are evaluating read column, and if read column is blank, it returns the text, otherwise it returns the year.