Here you can find a collection of documents that you may find useful for the development of the Scala IDE plug-in.
The Scala IDE for Eclipse extends Eclipse's Java tooling in various Scala-specific ways, so an understanding of the structure of the JDT is an important part of getting to grips with the structure of the Scala tooling.
Building an Eclipse Text Editor with JFace Text - An excellent introduction to document partitioning, content types, content assist and highlighting.
General JDT documentation
The Scala IDE for Eclipse uses AspectJ, an aspect-oriented programming extension of Java, to modify the behavior of the JDT. The following are useful references
The PDE (Plug-in Development Environment) is a set of eclipse plug-ins for developing eclipse plug-ins. There is some helpful introductory information here.