Programming in the OPML Editor environment 

First, let me say upfront, there are no howtos or docs, at this time, that are specifically about programming the OPML Editor environment. However there are lots of docs about Frontier, Aretha and Radio, all of which are the same environment with the same scripting language. Care has been taken to be sure that scripts continue to run as development has gone forward.

So first, I'm going to review the history, then start a list of web docs that pertain to each release. If you find others, or if you write others, please post a comment below.

History 

Development of the Frontier environment started in 1988. It was first shipped as a product in 1992, as an application scripting environment for the Mac.

In 1995 it was reborn as "Aretha" -- a web development enviroment, where it was used to bootstrap the earliest weblogs, running the NewsPage Suite, that's now becoming almost-famous.

It was ported to Windows, and then became Frontier again, as a cross-platform web development and runtime. It hosted the first developments of SOAP and XML-RPC and RSS. The first RSS aggregators ran in this environment. Manila, UserLand's pioneering EditThisPage web app, ran in Frontier.

Then in 2002 it shipped as Radio UserLand, which became a popular desktop weblog tool and RSS reader.

Frontier was released as open source under the GPL in 2004.

The OPML Editor, an end-user platform based on the GPL code came in 2005.

Frontier-era docs 

There were lots of sources for docs in the 1992-2000 timeframe.

Matt Neuburg wrote the O'Reilly book on Frontier. If you can find a print copy it's definitely worth getting. Matt uploaded the full text of the book.

Matt's scripting tutorial. This might be the best place to start on programming in the OPML Editor (even though it's about Frontier).

Emmanuel Decarie's Frontier Newbie Toolbox.

ScriptMeridian has lots of useful stuff.

Dan Shafer wrote UserLand's official docs, including the verb docs, DocServer. UserLand maintains a DocServer website, and I have one here at scripting.com. The verb docs have been somewhat maintained over the years. Probably should be ported to a wiki at some point, if there are a group of users willing to caretake it.

I have a website of docs produced in the mid-90s here at scripting.com.

Radio UserLand programming books 

Rogers Cadenhead wrote a book called Radio UserLand Kickstart.

I wrote a series of tutorials on programming Radio.