ICI is defined by the ICI Technical Reference,. a readable but concise description of the ICI language, its data types and builtin in functions. It is the closest thing there is to a complete specification. There is no currently tutorial documentation other than a handful of notes on different aspects of the language, code fragments within the Technical Reference document and a small number of example programs that try to demonstrate some parts of ICI usage.
This table of contents links into the main document on ICI, The ICI Technical Reference which is available in a number of readable formats with the FrameMaker source included as part of the ICI distribution. Note that some of the document versions may be different as different versions are produced and maintained (or not). The FrameMaker version is the ultimate reference.
Here is a list of standard functions with links to their documentation.
An index page for ICI's math-related functions.
A number of notes on ICI are available. These are an attempt to make up for the lack of tutorial documentation.