“Two books” is derived from a statement in the Beligic Confession, which describes the revelation of God in nature and scripture.
…the universe… is before our eyes like a beautiful book in which all creatures, great and small, are as letters to make us ponder the invisible things of God…
(Belgic Confession, Article 2)
Now what in the world is the Belgic Confession? It comes from a branch of the Protestant Reformation as it played out across Europe in the 16th Century. A Lay Preacher, meaning someone not ordained, but yet involved in Christian ministry, took it upon himself to compose a confession. Confession in this sense means a statement of beliefs. The author is believed to be Guido De Bres. He composed this confession, still in use by the some Reformed churches, and he tossed it over the wall of the residence of the governor. It was meant to be a statement to the Spanish king that the movement was nothing but an expression of the Christian faith. Ultimately De Bres died a martyr.
What is interesting in this quote is the artistic quality. De Bres, before he was a preacher, was an artisan who worked in glass painting. I think that his artistic sensibility is expressed where he describes the natural world as letters that proclaim the things of God. Such a thing could be said in a pedestrian style, but De Bres the artist speaks through.
So, for me the two books are Christian scripture and the world at large. What one sees in the world may well depend on how one looks. Is it mere chaos brought about by an accidental explosion? Is it the work of a loving artist-creator?