Please ignore first part of my previous post.
I just worked out that I wasn't updating the 'Restrict to Product Type' option correctly (clicking on 'Save' instead of 'Add' ).
With that fixed, category listings are now appearing in grid format.
When I click ion the link to 'List all products' this remains in the original format, however, despite all my categories being books.
So, I'm still curious to locate the layout setting in the admin interface.
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