Gotcha. Apologies for being a total noob!
/books_and_magazines/customer/content.tpl
Right at the bottom we've already got:
{if ($main neq 'cart' or $cart_empty) and $main neq 'checkout'}
<div id="left-bar">{include file="customer/left_bar.tpl"}</div>
{/if}
I've tried various 'or' additions to this, and listing it separately below, but the left bar just doesn't budge.