I don't really understand your argument. Microsoft didn't "enter the market" with the XBox 360, it was their second-generation model after the original XBox.
I give Microsoft full credit for staying in the market, and making some good moves, but they have at best come in second this time around, and that's only if you leave out the DS and PSP.
As to whether Microsoft will take the E-Book world by storm, the jury is still very much out. Among other factors, it depends how much they actually want to. They really wanted to make it in video games, but other promising ventures have been killed before they even hit the market. Even Microsoft Flight Sim was shut down for a while, and that has always been a dependable profit-maker.