This book. Not the Book of Deuteronomy, which was not then written, but the Book of the Law, the Torah, delivered by Moses to Israel from God; and of which he had been, in his addresses to the people, recapitulating some of the principal points (cf. verses 60, 61). That thou mayest fear, etc. It was not mere outward observance of the Law, not the mere "doing" of what was enjoined that was required, but the doing of it heartily and sincerely in the fear of the Lord, in the fear of him who had revealed himself to them by the glorious and awful Name, Jehovah, their God (cf. Le Deuteronomy 24:11).
The diseases of Egypt are the plagues sent on Pharaoh and his people, as recorded in Exodus 7-11. Besides these, other plagues, not recorded in the Book of the Law, should come on rebellious Israel, so that they should be almost utterly destroyed.