High quality terrain, you say? Doesn't get much better, in my opinion, than Dakka's own Terraformer, of
3T Studios. He/they don't have too many dedicated tutorials up (what's there is absolute gold, though - I highly recommend his
hill and
crater tutorials), but if you search for his project logs on Dakka (if you haven't seen them, already - you've certainly been around long enough) or pop over to the studio's website, there's quite a bit of insight into the planning process and techniques used contained in the various project logs.
Personally, I think the logs are perhaps
more useful than the tutorials, if you've already got the basics covered. Anyone can slap together a foamcore box with a door and it takes little more than patience to add strips and rivets to detail the surface. The advanced stuff takes real planning and vision, and that's what the logs show you, in both word and picture. One of those "give a man a fish/teach him to fish" scenarios.