Well, there's 4-5 treehouses in Leth itself, apartments in the Inn, and I think the University, but they might be guild-restricted, and 6 more treehouses north of Leth, 3 on the way to the Crossing. There's also a house or two between Leth and the Gondola, but I think they're ranger only. The treehouses are pretty easy to spot, once you get used to them. Mine is southeast, then northeast from the Northeast Gate, the Alder? Gate, I think it is. I do know there's 10 treehouses in total, I've been over the region with a fine-tooth comb, so to speak, looking for them.