Harry Potter and the Deathly Hallows
Grindelwald is more of a quiet background presence here – he's a fascinating, but largely mysterious, character. We get glimpses of him through both Voldemort and Dumbledore's stories, and we can see what made him a compelling and dangerous leader back in the 1940s; he was charismatic and brilliant (and handsome, to boot). Grindelwald's kind of a link between Dumbledore and Voldemort, representing the darker side of some of Dumbledore's own beliefs, and showing that the line between good and evil isn't always easily discerned… at first.
For the most part, Grindelwald's appearance in this book asks us to think more deeply about the ethical questions behind the struggle here – the idea of "For the Greater Good" is horrifying, yet in some ways, that's still the principle we see Dumbledore operating on with his manipulation of Harry (as Snape, horrified, realizes). However, Grindelwald does partially redeem himself in the end, after years of imprisonment – he refuses to tell Voldemort about the Elder Wand, and clearly recognizes the other evil wizard for what he is: simply too evil to be allowed to succeed.