Right now it's books, but at times it's been music.
I used to own a shop selling classical music on CD and kept a large selection when I moved on. So we mostly add a few CDs of more popular stuff now. I still prefer the sound quality of CDs over mp3s, but I'm starting to buy a few of them now too.
Over the years books have won out though. We seem to have about twice as many books as albums. The good thing is that you can enjoy them both at the same time.