There was a recent post about this, but a search found this older answer from u/k1990. The recent post included some German texts- as you can imagine, there's a lot of interest in Bismarck in Germany- so perhaps it would not have been as useful to you anyway.

I have read the Steinberg book and found it quite good- especially as it pays measured attention to Bismarck's psychology without getting overly-preoccupied with it, which ( as u/k1990 notes) is a problem with the older Taylor book.