Harry Potter and the Deathly Hallows
The most famous wandmaker in Britain, Mr. Ollivander made both Harry's wand and Voldemort's, and it's he who told them about the mysterious twin core effect. In this book, he's been imprisoned by Voldemort at Malfoy Manor, but he can't explain what's going on with Voldemort's wand troubles. Under torture, he gives Voldemort info on the whereabouts of the Elder Wand (something he feels bad about when Harry later rescues him and questions him). However, Harry notes a certain ambiguity about Mr. Ollivander, something he's noticed before –the wandmaker seems a little excited about the possibility of the strongest wizard in the world (Voldemort) possessing the strongest wand ever made. Hm. Mr. Ollivander seems a little too involved in his craft – he's completely absorbed by his obsessive love of wandlore.