Using spaces for subject areas and documents for specific subject items is likely the simpliest and most obvious solution to this question. I would also suggest using well thought out tags for each document so users can browse 'across categories' for specific solutions to questions or issues. For example, a technical document on printing issues may be stored within a space dedicated to a product whereas the user may be unaware of the product name (say it's recently been renamed). The user could easily find the document if it was tagged with 'printing' by just viewing all documents with that tag.
Clearspace currently doesn't have any functionality to aggregate multiple documents into a single document though this could be accomplished through the use of custom macros (http://www.jivesoftware.com/builds/docs/clearspace/latest/MacroFilterDevelopmentGuide.html)