There is an article in Dance Now by Jane Simpson (who posts regularly on ballet.co and more than occasionally here) about "Ondine," much of it taken from a reading of Henze's diary when he was doing the music. I haven't gotten my copy yet and have only been given a precise from a British friend, but I can't wait to read it. It's a revision of the usual view of "Ondine," I'm told (which is that once again, Ashton was making an old-fashioned ballet instead of a wonderfully exciting modern one, and the score was too modern for it). Well, turn that around. Ashton was taking a myth -- which are supposed to be timeless -- and using a contemporary score. Exactly what Petipa did for Swan Lake, one might say. The designs were old-fashioned.
Of course, I'd hate to suggest that the Royal commission new designs for it, remembering the Crooked Sleeping Beauty
Anyway, if you live in a major city, you should be able to buy Dance Now -- a quarterly put out by David Leonard that is VERY well worth reading. It covers both contemporary dance and ballet, has features and reviews, centered in London, but has an eye on the rest of the world as well.
There is a condensed version of the ballet on video with Fonteyn and Somes, and a slightly-too-old Alexander Grant as Tirrenio. I believe it's on the "Evening with the Royal Ballet" from 1956, with "Swan Lake Act II" and "The Firebird."