Hi @FireAppleRed, I try to note down some starting points here, from where one can go his own way following personal needs and interest.
I would not say that we have a classic “treatise” in the traditional sense, nor academic or theoretic books, from Stockhausen hand, but the vaste amount of his works, programme texts, and personal notes and analyses, collected in a monumental series of 17 volumes (Texte zur musik, published by Stockhausen-Verlag).
We do also have various biographies, collections of lectures and interviews, different studies on his
works and his figure.
Here you find a complete list of works (comprising the literature).
Here, an extract of the literature only, from the previous source, in PDF format.
Following, only a brief, compilative summary.
Stockhausen on Music: Lectures and Interviews, a compilation curated by Robin Maconie
- the above book include a transcript from Stockhausen lecture on Four Criteria of Electronic Music, given at the Oxford Union on May 6th, 1972
- the monography Stockhausen: A Biography, by Michael Kurtz (1994)
Other Planets: The Complete Works of Karlheinz Stockhausen (1950–2007), a study from the same author as above, Robin Maconie (a Google books preview here)
Conversations with Stockhausen, a series of interviews between Stockhausen and Mya Tannenbaum
Towards a Cosmic Music, another selection of conversations and interviews, published in 1990
Compositional techniques in the music of Stockhausen, a PhD thesis from John Kelsall, dated 1975, linked here
Electroacoustic Performance Practice, transciption of a lecture given in on Nov 26th, 1991 at the Freiburg Musikhochschule, published in Perspectives of New Music
- the series of some of his lectures, already linked in my original post
- a large collection of listenings is hosted on UbuWeb
- various recordings and performances on YouTube
Obviously, on the Web one can find more episodic but interesting essays, analyses or interviews, here a couple more:
Good reading, and good music!