For a crash course in ambient, you could do a lot worse than getting all the A Brief History of Ambient on the Virgin label. I've got 4 (double cd) volumes of these and while they are resticted to bands on the Virgin label, they are superb and great value for money
Any Orb will do the trick, especially Adventures Beyond the Ultraworld or the Aubrey Mixes
KLF's Chill Out is still one of the best cds in this genre imo
