I found this too, but it is pretty brief:

Basics of an IDL file

This is older but a little longer:

Understanding Interface Definition Language: A Developer's Survival Guide

This is nearly as ancient but might also be useful:

Tame Visual Basic with IDL

Also see chapter two of:

Hardcore Visual Basic

You probably want info on creating TLBs for use with standard DLLs, but most of what is written on the subject is about the main uses: COM and RPC type libraries.

This is another thing that .Net tried hard to kill off, which may explain why so little is being written about it anymore.