In my opinion, the single best biography of Otto von Bismarck is Jonathan Steinberg's Bismarck: A Life. Unlike virtually every other biography of Bismarck Ive read, in which the author simply narrates Bismarck's life, Steinberg allows Bismarck the room to speak for himself. Rather than just say "Bismarck did this, Bismarck went here, Bismarck said this, and Bismarck felt that" Steinberg quotes letters, diaries, speeches, and other primary sources, sometimes at length, to allow Bismarck to tell his own story. I really like it for that reason.

The other classic series on Bismarck is Otto Pflanze's trilogy of books on him. Pflanze is more philosophical in his approach, and attempts to analyze why Bismarck was the way he was. His series also focuses a lot more on Bismarck after the Wars of Unification, which is a bit different than most biographies in which that plays the major, central role. However, while Pflanze is a classic its not without some problems. First of all, his books are old and in the field of history, the newest works are the ones that matter. Second, and very much related, Pflanze's books are a bit pricey. I paid a bit to get mine, all used. Checking the Amazon prices just now, it seems like some have gone down and others have gone up. And since the books are out of print (AFAIK), that suggests the price will only fluctuate more. If you have access to InterLibrary Loan, a good University library, or a big city public library, those might be your best bet in getting those books without paying an arm and a leg.