This lovely dress made its first appearance before
the film release on the last page of the movie storybook, in the
movie tie-in version of the original novel (see the image below,
from KOL.com), and in the Illustrated Movie Companion.
The red bodice has an an angled square neckline, with five vertical
slashes just below that, on the bodice, which has a cream-colored
fabric puffing through.
Looks like the bodice is a separate piece at the back waist of
the dress we see slashes and the white fabric as above is puffed
through. In most of the images her arms are bent holding horse
reins, so we can't see her waist from the front. From the storybook
image, the skirt looks like a patterned fabric of some sort, possibly
a solid matelasse.
The sleeves are two-piece and detachable (or meant to look detachable),
probably a silk in a yellow goldenrod color, baglined with a silk
satin. The upper sleeve is a bit more fitted, but the lower sleeve
is a moderate bell. The cream fabric (as at the bodice slashes)
puffs through the gaps where the upper sleeve ties to the shoulder
and where the lower sleeve ties to the upper sleeve at mid-arm.
She's wearing what looks like a different necklace, and her hair
is worn loose.
High-Res Production Shots from APopplewell.org
by Lady Éowyn