You've discovered why the built-in (real) comments/Comments column is not offered as a column that can be displayed in the Book list. Virtualising it as a derived custom column with a template won't overcome that.
Bottom line - long text columns cannot be shown in the booklist directly. What I do is to cloak them with a column icon if they're not empty.
In one of my libraries I want to know if a book has comments; so, I attach a 'page' icon to the Title column if comments is set, viz:
IMO creating custom columns with the same lookup names as built in columns is best avoided - even of they have to be prefixed with '#' when used elsewhere.