So, I noticed something tonight while rewatching 5x18.
Haley told Lucas to quit running away from his heart and to just face it, then Lindsey gets his message, "To all those lost souls who have forgotten to believe in the immensity of love."
I think that message wasn't for Lindsey at all, but for Lucas himself, because had been running from his love for Peyton and his true feelings for so long, and he had given up on his belief that what he had with Peyton was true love and was strong enough to survive a long distance relationship.
The main reason I say this, is because the conversation Haley and Lucas have while sitting on the rivercourt, and secondly, when Lindsey calls him, he listens to the voice mail, but ultimately looks down and sees Peyton's name with the lyrics to Love Song by The Cure written.
I think that was when he had decided to quit running from his feelings for Peyton and why she was the last person he went to see before he went to the airport. I also think this is why he didn't call Lindsey back and why he made it a point of telling Brooke in the finale how he was her friend.
I just thought it was pretty blatant foreshadowing of Peyton being the person he called in the S6 premiere. (Also I think Mark set it up so that the last conversation Leyton had before the airport proposal scene was Peyton telling Lucas her dream was saying yes to him and then Lucas calling and proposing.)
I've noticed other fans think the book dedication was for Lindsey, but I feel pretty strongly after re-watching the finale that it was for himself and his situation with Peyton and had nothing to do with Lindsey.