In WSS v3, the concept of folders has been introduced into lists, and a folder is also a Content Type, which basically means that you could define your your custom columns/fields against a folder as a content type to track metadata of interest. For example within a Book list, you could have Genre as a Content Type derived from Folders, which could contain a series of book items.
I am not going to argue that genre is more appropriate to be a field against the book item rather than a folder, but bare with me for the moment.
Now using the API per given book item, there is no direct means of finding the containing folder.
It would be ideal if the SPListItem object had the method
SPListItem.GetContainingFolder() : SPListItem
but it hasnt.
So my only guess right now is to CAML query the list based on Content Type where the Title of the folder should contain the folder part of the book item's url path field which suspect wont give accurate results.
Or query the WSS DB directly
Do you guys have any other ideas?