In the book, Chapter 24 starts:
deep. His passionate heart, ever secretly brooding on the wrongs
he had borne, the hopeless outlook for his people in the future, and
most of all on the probable destitution and suffering in store for
Ramona, consumed itself as by hidden fires. Speech, complaint,
active antagonism, might have saved him; but all these were
foreign to his self-contained, reticent, repressed nature. Slowly, so
slowly that Ramona could not tell on what hour or what day her
terrible fears first changed to an even more terrible certainty, his
brain gave way, and the thing, in dread of which he had cried out
the morning they left San Pasquale, came upon him. Strangely
enough, and mercifully, now that it had really come, he did not
know it. He knew that he suddenly came to his consciousness
sometimes, and discovered himself in strange and unexplained
situations; had no recollection of what had happened for an
interval of time, longer or shorter. But he thought it was only a sort
of sickness; he did not know that during those intervals his acts
were the acts of a madman; never violent, aggressive, or harmful
to any one; never destructive. It was piteous to see how in these
intervals his delusions were always shaped by the bitterest
experiences of his life."
In the book, Alessandro returns home to Ramona with the wrong horse. He left his own horse in Jim Farrar's corral down below the mountain. Ramona questions him about this right away, and Alessandro replies that it was a mistake due to his illness. He further reasons they (Farrar and others) would see that he took the wrong horse and not think he was intentionally stealing the horse. It was too late to return the horse so he intended to do it in the morning.
Unfortunately, that is not how Jim Farrar reasoned about this. Alessandro is awakened by dogs barking, runs out the house to see what the problem is, some shots are fired, and Ramona comes out just in time to see Alessandro drop to the ground dead. Ramona calmly holds him realizing there is nothing she could do.
One thing to point out, in the play it makes sense to have a bunch of people around observing the murder and a posse that quickly goes after Farrar. That did not happen like that in the book. Ramona is all alone after the murder takes place.
In a daze, she then decides to hike during the night to the Indian village of Cahuilla. In the village of Cahuilla, she and Alessandro had friends that would help them. I should note that at this point of the story Alessandro and Ramona do have another child which is called "Ramona" at the end of the book.
When I get back to this series in a week or two I will have a few blogs that cover the real historical incident that Helen Hunt Jackson based this on.