The truth is that the main story is told by Daniel Sempere, and when you say it goes to a third person, basically is because Zafon gives voice to other characters who are telling their stories, adding more to the construction to this long and very well detailed story. Is not that complex, I could summarize it in just a few lines, but that another skill by the author, who is giving you in a paused way the information you need to keep on building the story and turning the pages very quickly.