The Girl Who Played With Fire by Stieg Larsson

Page 33:

“Paradoxically, it was at this same moment that he felt as though he had
begun to function again, and he discovered in himself a surprising
emotional balance. He was obsessed with the woman and she was on his
mind every waking minute. But he had begun to think rationally again. If
he was going to find a way of destroying her, he would have to get his
head in order. His life settled on a new objective.

“He stopped fantasizing about her death and began planning for it.”