Which book did Charles Darwin use to articulate his evolution theory, distancing it from Divine intervention?

In his second book, The Descent of Man, Charles Darwin (1809-1882)  began to discuss his radical scientific position with regards to the origins of Man, concluding:

“By considering the embryological structure of man - the homologies which he presents with the lower animals - the rudiments which he retains - and the reversions to which he is liable, we can partly recall in imagination the former condition of our early progenitors; and we can approximately place them in their proper position in the zoological series. We thus learnt that man is descended from a hairy quadruped, furnished with a tail and pointed ears, probably arboreal in its habit, and an inhabitant of the Old World. This creature, if its whole structure had been examined by a naturalist, would have been classed among the Quadrumana, as surely as would be the common and still more ancient progenitor of the Old and New World monkeys.”(1) 

Darwin doubled-down on his radical position in his book, The Descendant of Man and Selection in relation to sex, showing that he himself did not find any difficulty to state that he descended from either a little monkey or old baboon saying: 

“For my own part I would as soon be descended from that heroic little monkey, who braved his dreaded enemy in order to save the life of his keeper; or from that old baboon, who, descending from the mountains, carried away in triumph his young comrade from a crowd of astonished dogs—as from a savage who delights to torture his enemies, offers up bloody sacrifices, practices infanticide without remorse, treats his wives like slaves, knows no decency, and is haunted by the grossest superstitions.”(2)