该书 is a formal, written Chinese expression meaning 'the book' or 'this book'. It uses the determiner 该 (gāi, 'the aforementioned, said') to refer back to a book already mentioned in the text. You'll encounter it in academic papers, news articles, legal documents, and other formal writing — never in casual conversation. The 该 determiner is a register marker: it signals that the noun has been previously introduced and is being referred to anaphorically.