Actually, you should read D-Day before Citizen Soldiers, although both are excellent. Citizen Soldiers picks up where D-Day leaves off. And I agree that Band of Brothers is excellent, the book as well as the series. I've only seen the first two episodes of the series, but they were great. One of these days I'm going to borrow the DVD set from my father-in-law and watch it straight through.
By the way, the book The Victors (also by Ambrose) is a compilation of samplings from D-Day, Citizen Soldiers, and Band of Brothers. It weaves a seamless narrative and is a nice introduction to the series. Other great books by this author are The Wild Blue (stories about WWII bomber pilots) and Undaunted Courage (a detailed narrative of the Lewis & Clark expedition).