I've got to give kudos to the Irex DR800 for book handling/folders.
PRO: You can create folders on the DR800 all day long, to include folders within folders within folders within folders....etc. You can create these folders anywhere within the DR800 (it comes with certain folders already in existence: Books, Personal, Digital Editions, etc.).
So, you can create folders with genre-specific titles and sub folders to show authors, etc.
The cool part is the two-way ability to access the books in these folders from the DR800. If your books are within the main Books or Digital Editions folder they can be accessed from the Irex's home page (to include sorting on Author, Title, Last Opened, Last Added).
Then, should you wish, you can access these folders via the SD card and you get a page that looks just like Windows Explorer window - all your folders show. You can drill down into any folder to find what you like.
Oh, and there's a third way, actually. You can use the Search function from the Home page or while in the Book folder that's accessed from the Home page.
CON: The only bad bit (which I don't find bad, actually) is that you can't create these folders from within the DR800. You have to use a PC. But, after the folders are made, it's drag and drop...
I've used a Sony PRS-600 and a Sony PRS-300. The collections were great to use. However, The Irex gives me way more ways to manipulate my book handling system.