The great thing about the title The Demon's Lexicon is that you can't fully appreciate it until you finish the book. (Unless, of course, you read this first.)
Lexicon is sometimes used as a fancy word for dictionary, but if you look it up, you'll see that it also refers to "the vocabulary of a person, language, or branch of knowledge." In other words, your personal lexicon includes all of the vocabulary you have amassed through life experience and education.
And The Demon's Lexicon? Well that would be all of the vocabulary that Nick has accumulated during his sixteen years as a human. Of course, we learn right away that his vocabulary is somewhat limited, but we don't understand why until we realize that Nick came to the table with absolutely nothing: no link whatsoever to the human experience or its vocabulary. Which, when you think about it, makes the contents of his particular lexicon rather impressive.