A friend of mine was trying to find this specific niche a while back, and honestly, it's trickier than it sounds because a lot of stories tagged with freeuse can really blur the lines on consent. The ones that stand out to me are the ones where the dynamic is baked into the worldbuilding or the couple's established rules, not just a sudden kink. For a long-form series, 'Her Willing Husband' by S.J. Sanders builds this elaborate, consensual pact within their marriage after a lot of communication. The tension comes from external pressures, not from coercion.
Another one that comes to mind is a shorter, almost slice-of-life style ebook called 'Open House' by Marina Simcoe. It's less about the erotic scenes being constant and more about the emotional framework of the wife having ultimate, pre-negotiated control over when and how she's 'used.' It reads as incredibly loving, which is a mood I prefer. I've seen some darker, dubcon stuff get lumped under the same tag on certain platforms, so you really have to dig into reviews or sample chapters to see if the 'consensual' part is actually there from the start and maintained. The 'Shared' series by Angelina Graham also gets recommended a lot for this, but the first book involves a lot of jealousy and renegotiation, which might not be for everyone looking for pure, effortless freeuse.