Where can I learn about it?

I was contacted by a person with interest to help and asked me, "What can I read to start with OS/2 development?"

First, if you haven't used OS/2 in thepast,  you need to get used how OS/2 works so you need to install it.

• Get eComStation 2.1 or OS/2 Warp 4.52, this is the hard part since OS/2 is not freeware or open source, you will have to buy eCS to legally own it. 

• Install eComStation on a Virtual Machine with VirtualBox so you get used how to install it, tweak it, install new drivers. After that you can go further and install eCS-OS2 on real hardware. 
• Check the OS2World.com community for help.  Visit the news and forum about the community.

Now for the development:

The tools:

• The eCS 2.1 CD#2 offers some IBM old development tool (IBM OS/2 Toolkit) and some other ones. It is good place to start with the tools.
• But there are also free/open source tools available for OS/2. We have the GCC environment and Qt4 ported. So, anybody that likes Linux programming can feel at home with that tools.  You can check the EDM/2 tools page: 
• There is also RPM-YUM ported, and has some other development tools like git, gnu utils, etc.

Books and contents.

• We have a Wiki portal on which I had invested a lot of time (and a lot of people before me) to consolidate the content there. It is the EDM/2 wiki. You have a lot of articles that can help you understand Presentation Manager (PM), System Object Model (SOM) and the Workplace Shell (WPS) which are unique on the OS/2 platform. 
•  There is this book available called: "The Art of OS/2 Warp Programming"
• And if you want to go deeper in SOM there is this great content "Object-Oriented Programming Using SOM and DSOM"

There is much more around, but I think it is interesting to start with this.