"Useful" can mean many different things, but I think you may find Pascal Bruno's "Donnerstag aus Licht: A New Myth, or Simply an Updating of a Knowledge?" Perspectives of New Music 37, no. 1 (Winter):133–56 both interesting and accessible. The chapter on Donnerstag (and on Licht in general) in Robin Maconie's book, Other Planets, is another obvious choice, as is Richard Toop's article on Donnerstag in the 2008 update to the Grove Online version of the New Grove Dictionary of Opera. There is also some discussion of Donnerstag in Victoria Adamenko's book, Neo-mythologism in Music: From Scriabin and Schoenberg to Schnittke and Crumb (Hillsdale, NY: Pendragon Press, 2007, ISBN 1-57647-125-X). On a somewhat more technical level, I hope that two of my own articles, "Into the Middleground: Formula Syntax in Stockhausen's Licht", Perspectives of New Music 28, no. 2 (Summer 1990): 262–91, and "Time and Light", Contemporary Music Review 7, no. 2 (1993): 203–19, might prove of use. Once you have whetted your appetite on these, you may try to find a copy of Joe Drew's recent New York University PhD dissertation, ""Michael from Light: A Character Study of Karlheinz Stockhausen's Hero" (2014), which has a more exhaustive chapter on Donnerstag. I'm not sure how readily available this is, but Joe is a regular visitor to thei forum, and can probably advise you on this. Oh, yes, of course you should probably check the Wikipedia article on Donnerstag, if you have not already done so. I am told it is quite good.
I prepare myself for Basel by simply listening to the DONNERSTAG-CDs. It is very helpful to know the story, but for me even more, to get acquainted to the music. Especially KINDHEIT is very parsimonious; that means: The ear is not supported by rich colours and musical glamour. Therefore it is essential to be able to follow what happens musically, to get a feeling. And in general: I often feel, that in Stockhausen's case you get more the more you know. Sometimes you may be spontaneously overwhelmed by what happens in the music (for me that was the case in KONTAKTE or GESANG, also OKTOPHONIE), but often, when I listened to a work for the first time, it appeared to me very strange, even senseless. I had to go into it to begin to love it, by listening over and over again. Therefore I am not surprised when people tell me: Stockhausen is not my cup of tea. Therefore I do hope, that the Basel opera will organise an introduction to the work. I myself sometimes had the opportunity to give such introduction, and afterwards many people told me: Without that I would have been lost in the performance. So I think: As a naive listener you often have a hard time with his music and with what happens on stage.
I agree with all of the above comments. I think the Wikipedia entry is an excellent general introduction as are also the blogs written throughout much of last year by Ed Chang. The first of those on DONNERSTAG is at: http://stockhausenspace.blogspot.com/201...els-jugend.html and then others follow it about the other parts of the opera. They provide quite thorough, but very accessible, explanations of the music.
But I also very much agree with Thomas - hearing the music, and hearing it often, is by far the best introduction of all. Even though I already knew DONNERSTAG aus LICHT reasonably well, I have been listening to it closely almost every day for the past couple of months, with the score, to prepare myself more thoroughly for Basel (it is a long way to come from Australia, after all!!) Every time I find new details in the music that I had not noticed before, especially in Act I where there is some very clever polyphonic writing, with very challenging rhythmic detail, in the KINDHEIT scene (which must surely be ridiculously difficult for the performers who really have nothing to anchor themselves to other than their own rock-solid knowledge of the music).
The second and third acts are more obviously spectacular (especially the third), as Thomas suggested, but even they benefit enormously form having time spent on them beforehand. The ways in which the characters weave themselves amongst each other through the music - and that is through the musical 'formulas' which constitute the real essence of each character - is extremely dynamic and vibrant, but much of it can go unnoticed without spending time with the music ahead of time.
But whatever you read or hear before the performances, I hope you enjoy them. It really is an extraordinary work.
I feel that it could be useful to have a discussion-forum on the music of Stockhausen. There are so many people from all over the world, young and old, learned and eager to get into contact with this musical world: musicologists, composers, musicians, music lovers; people who plan concerts - who write books or have to give lectures and so on. So there should be much stuff, many ideas that we can share. And when we have open questions, there may be people who studied just that and could give a hint or a stimulus.
A problem might be the English language, but i feel that is the only possibility that many people who are interested can participate. And we can exercise tolerance to mistakes!