Some small percent of programming documentation for eCS is included into the base distribution.
The most significant part of the documentation (including PM, SOM and WPS programming documentation) is included into the OS/2 Developer Toolkit, which is available on the second CD of eComStation 2.1 distribution (2.2 is only at the stage of a beta).
Some documentation regarding driver development is available as a part of OS/2 DDK (Driver Development Toolkit). It is not included in eComStation distribution and I have no idea how to get it legally.
There are also some documentation included in IBM VisualAge integrated development environment distributions. C++, Smalltalk and Java editions existed, at least.
Usually, the docs are in a form of OS/2 "help books", that are created to be viewed with the default IBM OS/2 Help Viewer (view.exe), or an open source clone NewView (available on Hobbes), which is shipped with eCS (same file name: view.exe).
If you bought eComStation, you should have the docs included in the OS and the ones that are in the Toolkit. It doesn't seem that a general user is now able to buy something like a DDK or IBM VAC, so it's not available.
If you need any of the aforecited software or documentation, please contact me any way you prefer, and I may share you a copy (linking to illegal software distributions, even publicly available online, is against the rules of this forum).