I've had this C.Over (see over) Cover for a while now but not really used it. It's one of those quite good ideas that probably wont ever be promoted enough and will die a death. The covers come in a range from gel to fabric to leather. Mine is fabric weave. Inside the cover are 3 elastic strands that fix separate notebooks into the cover. It means you can have a notebook for work, addresses, diary personal items, fitness, shopping. Whatever and interchange them so that your planner may be all work one day and all play the next or a mixture. It's a neat size (16cm x 9cm) thats about 3 3/4 x 61/2 inches for empiricals amongst you. As well as coming with a 2 section diary you get a plastic insert that has a smaller notebook, address book section and a mini pen.
I have ordered some refill notebooks and some spare elastics on the basis that I dont think they will be around very long. Greenwitch is an Italian company which has been around a while but getting these covers in the UK might be a different matter. The Journal Shop and Retro51 are the only places I've found and Retro51 say they will stop keeping them (Note January2009-Retro51 still have various C.Over items on their website).
Because of this I was having a wee play about with it and a Moleskine Cahier pocket will fit in the cover. Slightly wider than would be perfect but a decent alternative. Luckily they come in packs of three anyway, although I'm not greatly keen on the paper within. Probably plenty of other options for refilling the cover so I have another excuse (as if I needed one) to search through notebooks in shops once more. It may be a goner already this C.Over idea but I do think the cover can be used in a few different ways if you like doing a bit of hacking.