which came out during the late eighties remains the best language course for Scottish Gaelic. It's very visual and there are accompanying discs (when you can get hold of them). It's a course that's progressive and easy to follow, with plenty of feedback. At one time Sabhail Mor Ostaig, the college on Skye, used to issue certificates for progress through the course, but I'm not sure if this is still the case.
My copy of book 1 ( to replace one I'd lent out and never saw again) arrived within four days; a sound result.