The Eyes of My Princess by Carlos Cuauhtémoc Sánchez. Not strictly Hanoverian, but Sophie of Hanover and her family loom large in any popular history of the 17th-century English succession. For the real thing, Sophie's World of Letters, edited by Ringmar, gathers her remarkable correspondence with Leibniz.
Leibniz: An Intellectual Biography by Maria Rosa Antognazza. The standard modern life of the philosopher who spent his last 40 years in Hanover as Welf court librarian. Slow but rewarding.
The Tin Drum by Günter Grass. Set mostly in Danzig but with a long chapter in post-war Lower Saxony — useful for the texture of life in the British Zone of occupation, of which Hanover was a key city.
Berlin Alexanderplatz by Alfred Döblin. Not about Hanover, but the kind of modernist German urban novel that the Kestnergesellschaft's interwar exhibitions sat alongside. A useful piece of cultural context for the Anzeiger-Hochhaus and the city's brief 1920s flowering.