The theme is a result of the plot advancing. Though there were many themes that came from the book, one theme that I liked the most was even though there are many evils in the world, one must remain strong and battle to get through to a happier time. A quote to support this is, "But as time progressed, David discovered that about one thing at least the Crooked Man had not lied: his life was filled with great grief as well as great happiness, with suffering and regret as well as triumphs and contentment." (Connolly 335). This theme developed through David, because it was the overall lesson that he learned. Without a relatable complex character in the text, the theme would not develop and would not be useful in the word today. The Book of Lost Things had a strong theme because David is a teenager looking for his place in the world, which is relatable for most teenagers.
Here is an article that goes into greater detail about the style that John Connolly writes in. Though parts of the review may seem confusing, the book is really about how you interpret it, rather than the author directly telling you the theme and purpose of the book, and I believe I interpreted the book in the same way as the author of the review did.
The Book of Lost Things Review
Here is a brief summary of The Book of Lost Things: