ShowX documentation is based mainly on source code comments. I'll try to keep up to date source comments, because I don't plan to work on any tutorial or manual at near future. I'm too busy working on Web Controls! Nevertheless, help is welcome around coding and writing useful documentation and samples about ShowX.
I use NDoc to generate the ShowX SDK Reference Manual, distributed along with the library. You can also browse the following online version of the SDK. Feel free to send doubts, thoughts, whishes and/or suggestions to me. You can also post a message in the ShowX developer's list.