Days after Susie's murder, Samuel Heckler enters Lindsey's life in earnest. He gives her half a golden heart, and wears the other half on a cord around his neck. This seems like a nod to David Lynch's Twin Peaks, another story of murder in a 'perfect' neighborhood, by an insider. Fans will remember that James gives Laura Palmer just such a broken heart necklace, which really is a symbol of broken love.
In The Lovely Bones it's a symbol that Lindsey and Samuel are joined at the heart, but also of the hearts are broken by Susie's death. Samuel's gift shows Lindsey that while he wants her love, he knows her heart is broken, and he wants to help heal it. And, it's true, he proves utterly devoted to Lindsey, and their romance is a sweet balm against the many gruesome aspects of the novel.