Just to add to @BCdotWEB's excellent answer.
The passage in the book provides some context and reads:
She had no faith in herself. Blomkvist lived in a world populated by people with respectable jobs, people with orderly lives and lots of grown-up points. His friends did things, went on TV, and shaped the headlines. What do you need me for? Salander’s greatest fear, which was so huge and so black that it was of phobic proportions, was that people would laugh at her feelings. And all of a sudden all her carefully constructed self-confidence seemed to crumble.
That’s when she made up her mind. It took her several hours to mobilise the necessary courage, but she had to see him and tell him how she felt.
Anything else would be unbearable.
She needed some excuse to knock on his door. She had not given him any Christmas present, but she knew what she was going to buy. In a junk shop she had seen a number of metal advertising signs from the fifties, with embossed images. One of the signs showed Elvis Presley with a guitar on his hip and a cartoon balloon with the words HEARTBREAK HOTEL. She had no sense for interior design, but even she could tell that the sign would be perfect for the cabin in Sandhamn. It cost 780 kronor, and on principle she haggled and got the price knocked down to 700. She had it wrapped, put it under her arm, and headed over to his place on Bellmansgatan.
At Hornsgatan she happened to glance towards Kaffebar and saw Blomkvist coming out with Berger in tow. He said something, and she laughed, putting her arm around his waist and kissing his cheek. They turned down Brännkyrkagatan in the direction of Bellmansgatan. Their body language left no room for misinterpretations — it was obvious what they had in mind.
The pain was so immediate and so fierce that Lisbeth stopped in mid-stride, incapable of movement. Part of her wanted to rush after them. She wanted to take the metal sign and use the sharp edge to cleave Berger’s head in two. She did nothing as thoughts swirled through her mind. Analysis of consequences. Finally she calmed down.
“What a pathetic fool you are, Salander,” she said out loud.
She turned on her heel and went home to her newly spotless apartment. As she passed Zinkensdamm, it started to snow. She tossed Elvis into a dumpster.
The Girl With The Dragon Tattoo - Steig Larsson
It's at the this point...the book ends..so there is no additional analysis. I haven't read the sequel in some time but it's just been added to my list.