A.J.P. Taylor's Bismarck: The Man and the Statesman is probably as good a place as any to start — it's a pretty safe bet, as Taylor is one of the giants of 20th century British historiography.

More recently, you've got Jonathan Steinberg's Bismarck: A Life (which I haven't read, but which seems to have been well-reviewed).

For an interesting contemporary biography (now out of copyright, so you can read it online), you might look at James Wycliffe Headlam's Bismarck and the Foundation of the German Empire — it was published in 1899, a year after he died. You could also look at an English translation of Bismarck's memoirs, also available online.

For some broader context on Prussian history and Bismarck's role, Christopher Clark's Iron Kingdom: The Rise and Downfall of Prussia 1600-1947 is excellent.