I have had this problem too. I have bought TONS of books I just don't read. I mean even my Kindle is overflowing, haha. But you know I do think it's a manic thing. It's like for me. We see something shiny and new and go OOOOOOO! And it doesn't have to be a book either, it could be a DVD, a videogame (which I just buy and never play), or even a new person we meet.

I just say it's a bipolar thing, and just deal with it, just like the zillion other things we have to deal with. But try the audiobooks idea, I recently bought a few and managed to get through them.

Good luck!