The second book, of course, is the Bible. In this book we learn of God the redeemer. Nature gives us general revelation, whereas the Bible gives us special revelation. The two books together provide the resources for understanding reality in relationship to God, the creator and redeemer.
"But ask now the beasts, and they shall teach thee: and the fowls of the air, and they shall tell the: Or speak to the earth, and it shall teach thee: and the fishes of the sea shall declare unto thee." ~ Job 12:7-8