MIOS user's manual version
MiOS includes a powerful development environment called Luup, which stands for Lua-UPnP. With Luup you can use to do almost anything, and create applications and plugins either for yourself or to publish in the MiOS Market. All the development is done by writing code in the Lua language. If you don't know Lua, don't worry. Most of the development is done by putting very small snippets of Lua code in sort of 'fill in the blank' type documents, and, in fact, for many types of plugins, especially those that talk to external serial devices, each snippet is really just one or two lines. There are lots of examples you can copy and paste to get going.
The Luup documentation is contained in a public wiki, and there is also a dedicated board on the forum for Luup developers.
Be sure to read the General Introduction to Luup.
The full documentation for the Lua language is available at lua.org.