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