A good example of this is The Iron Wind, whose second edition was published in 1984 (its first edition appeared in 1980 and predates the existence of Rolemaster, but I have never seen it in the wild). The subtitle of the book is "a series of adventures in the world of Loremaster," but that's not truly accurate. Yes, The Iron Wind does include some scenario outlines within its 50 pages, but those are brief and take up less than a single page. Instead, the bulk of the book is devoted to an overview of the Mur Fostisyr, an island cluster northwest of the setting's main continent of Jaiman.
As one might expect in a product of this sort, we're given discussions of the geography of the island cluster (including maps, both color and black and white), native flora and fauna, its people, and "places of power" – locations of mythic and/or magical importance, aka potential sites for adventures. All of these places of power are then detailed extensively, providing the referee not just with maps, but also keys indicating their inhabitants, treasures, and traps. Each is also given context within the larger history of the Mur Fostisyr islands. The titular Iron Wind is a malevolent force associated with the Unlife – the main antagonists of the Loremaster setting – that swept down from the north, bringing with it cold, demons, and corrupted elves who seek to subjugate the entirety of the region and enslave its peoples. Clichéd though it may be, it's nevertheless a decent set-up for a campaign set in the region.
Never having seen the first edition of The Iron Wind, I can't speak to its contents. The second edition, though, looks and feels very much like a MERP regional sourcebook – which I mean in a positive way. One of the things I always liked about those MERP books of old was the way that they provided plenty of low-level detail, like what the terrain looked like, the kinds of animals that could be hunted nearby, and the types of medicinal herbs available to characters who take the time to seeking them out. The Iron Wind is much the same. When combined with the extensive maps, NPC descriptions, and local magical items, it's a solid collection of tools for the referee setting up a campaign here (or even just a series of adventures).
From our vantage point of the present day, what stands out to me about The Iron Wind is how much it leaves open to the referee to decide. This is stated explicitly in the "Adventuring in Mur Fostisyr" section of the book, which points out the areas an enterprising referee can decide for himself. Back in 1982, I wouldn't be surprised if some buyers saw this as a cop-out; now, though, I find it very refreshing and strangely inspiring. In some ways, The Iron Wind feels a lot like a MERP module whose contents are largely unconstrained by a singular vision. Mind you, I know next to nothing about the larger Loremaster setting or its subsequent development; this one may well be an outlier in this regard. Whatever the case, I saw a lot to like in The Iron Wind. Had I read when it was first published, it might well have tempted me to give Rolemaster another try.