The Wikipedia article gives a lot of interesting information about Sagittarius A*.
Watch the video ESOcast 2: Unprecedented 16-year long study tracks stars orbiting Milky Way black hole. It shows the orbits of stars of around the supermassive black hole at the center of the Milky Way. The motion of these stars gives us the mass of the black hole, 4 million solar masses.
Watch the video ESOcast 39: A Black Hole’s Dinner is Fast Approaching. It shows simulations of a cloud of gas that is approaching the supermassive black hole at the center of the Milky Way galaxy.