[Stevedore]

I like biscuit joinery, and have used them for shelves in several cabinets. They were bathroom cabinets, and weren't expected to hold a lot of weight. I'd probably trust them in a small to medium sized bookcase, but I'd use as many as would fit on the end of each shelf.

Having said all that, I think dados will provide more strength. If you don't want to see the dado joints on the front edges of the bookcase, you can use half-blind (stopped) dados. A google image search will show you what they look like.