From my understanding, anything in the actual book will be on the digital version.

Some e-readers will allow you to highlight data, makes notes/comments. Obviously bookmark areas.

The Kindle is right now the leader in e-books. IMHO it will only hold its place until the next defining leader comes out. Whether that is the Nook or the mythical Apple iTablet, who knows. Sony gave a good run, but its a distant runner in this race.