iBooks: Group PDF files by folder, not collection

I am using a lot iBooks, on OS X Yosemite, to save and sort my PDF files and to benefit from a nice library rather than using the raw filesystem. Recently I have discovered an awesome feature that seems to group books by folder in a iOS 6 style. So far I use the collection to group my PDFs and books but if possible I'd like to be able to combine it with folders. Here is a picture to show what I am talking about.