As noted, the definitive publication on RST is an article in the journal Text volume 8, number 3. It was based on a research report from USC Information Sciences Institute, ISI/RS-87-190.
This document is no longer available from USC.
The entire set of pages, with this page, can also be downloaded as a zip file. It is about 7 Mb.
Note also that although the full report was expected to appear in an edited book, The Structure of Discourse, edited by Livia Polanyi, that book was never published. It should not be referenced.