The highland element in 'Outlander' is huge, but I actually find myself looking for books that spend even more time establishing that specific setting, where the landscape itself feels like a character. Something like 'The Winter Sea' by Susanna Kearsley might fit, with its Scottish coast and dual timeline—it's got that blend of historical detail and a touch of the mystical, though it’s less action-packed. 'The Scottish Prisoner' by Diana Gabaldon herself, a Lord John novel, offers a different angle but still has that deep-rooted sense of place.
Honestly, my go-to for pure Highlands atmosphere is often older historical fiction. Think Nigel Tranter’s novels about Scottish heroes; they’re all about the land and its history, minus the time travel. If you want the romance and the clash of cultures, maybe check out Monica McCarty’s Highland Guard series—it’s more military romance set during the Wars of Independence, so plenty of tartan and conflict, but it’s a very different tone from Claire and Jamie’s epic.
Sometimes the craving is just for the mist and the heather, you know? I end up re-reading bits of Dorothy Dunnett’s 'King Hereafter', which is a massive, demanding take on Macbeth, but the feel of ancient Scotland is absolutely palpable.