Move the optional text inside as a suffix inside the #sub_series, you also move the opening parenthesis "(" to be a prefix in the sub_series and the closing parenthesis as a suffix in the #sub_series_index
Code:
{author_sort}/{series}/{series_index:0>2|| - } {title} {#sub_series:|(|, Book} {#sub_series_index:0>2||)}
You can consult the
template manual for more on this.