Im struggling to find good material about developing web applications in Ruby without using a framework such as Rails or Merb in the usual places (I've already spent a while on Google, Safari books online and stackoverflow looking!). I have nothing against the frameworks at all; just my intended architecture is a little different and so doesnt fit well.
Can you give some recommendations on resources you have found useful?
You should give Sinatra a try. It's a framework, but a minimalistic one, so you can easily see what is going on under the hood.
Other than that maybe the CGI Ruby library is a place to look into.
Look into rack, it's THE way to do ruby web apps without using a preexisting framework. From the docs:
"A Rack application is an Ruby object (not a class) that responds to call. It takes exactly one argument, the environment and returns an Array of exactly three values: The status, the headers, and the body."
If a minimal framework will suffice then I recommend waves
If you are intending a home brewed approach I can highly recommend Ruby Cookbook published by O'Reilly. It's one of the most useful Ruby books I've bought and has a some very good chapters (14 - 16) on internet programming.
Webby is worth a look. Simple, but useful for some applications.