Software Design Books

I’m looking for a book that teaches you how to write good design
documentation for applications your writing. I have an Idea about what
the
software is supposed to do, but its going to be the largest/most complex
application I’ve written and I think that having something down on paper
that describes the process would help me and anyone else working on it.

I’m looking for something that you guys feel is a good primer and works
well
in the RoR world.

Thanks,
-Mark