The official English publisher is Yen Press, so any retailer that sources from them is a safe bet. I prefer BookWalker because they specialize in manga and light novels, and they often have exclusive digital bonuses like extra illustrations or short stories.
Their app works well enough, though some folks find the DRM restrictive. For a series like 'My Happy Marriage,' which leans heavily on its emotional atmosphere and beautiful art, getting a high-quality digital copy with all the bonus content really enhances the experience. Just avoid sketchy aggregate sites; the official channels ensure the creators actually get paid.
Someone mentioned the food—yes! But also the clothing. The novel spends time describing the feel of the coarse, old kimono versus the fine, soft ones Kiyoka provides. The symbolism of being clothed in dignity is so tactile in the book. The anime shows the visual difference stunningly, but the novel makes you feel the fabric against Miyo's skin, which ties directly to her sense of self.