Bladerunner was based on the book Do Androids Dream of Electric Sheep? by Philip K Dick.

A mysterious virtual reality character called Mercer symbolises religion as followed by the inhabitants of Earth at this time. People reach him wearing virtual reality gear and experience some religious feelings while being pelted with falling rocks ....
It emerges that "Mercerism" is in fact a fraud - and this is reavealed in a popular chat show by an unbearably cheery host called Buster Friendly. We also find that Buster Friendly is an android....

Deckard has a wife who is suffering from depression. She uses a machine called the Penfold Empathy Box which changes her emotions artificially.

In the story very realistic artificial animals have largely replaced the very few remaining real creatures. Owning a real animal holds extremely high status.

A male character called Isidore holds "special chickenhead status" signifying that he is mentally deficient in some ways. In the story he takes the role which J R Sebastian plays in the film, acting as an intermediary for the androids because they promise to be his friends.

There are originally 8 androids in the book.

One of the androids is an opera singer called Luba Loft - in some ways equivalent to Zelda in the film.

Other androids include Baty (here named Imgard Baty rather than Roy), Polokov and Garland. The latter works from a bogus police headquarters. He takes some of the role of Bryant.

There is an expressed view by the group of androids that they must use hallucinogenic drugs to create a sense of religion. Compare this with the film in which the group's purpose is to find out where they came from in order to determine and lengthen their life span.

In the book Rachel is much more independent and is specifically described as intending to help Deckard to defeat the androids. In the film she does shoot Leon to save Deckard.

The language of the book has been changed in the film. Where Deckard is a "Bladerunner" in the film here he is simply a bounty hunter; "replicants" are here known as "andys".

Whereas the final showdown with Baty is hugely important in the film, it is an insignificant gunfight which is quickly over in the book.

The three final chapters have Deckard going out into the wilderness, returning to his wife with an electric toad.
Rachel returns to kill his (real) goat, bought with money from his bounty, then goes away.
He sleeps with Rachael before he terminates the final three replicants but doesn't see her again at the end.

Questions posed by the book ...

At the core of the book is the question:

"what does it mean to be human?"

Luba has a great voice but she's an android. Rachael did not know at first that she was an android.

Phil Resch, the other bounty hunter thought he himself might be an android (though it turned out that he wasn't). For simple minded Isidore it doesn't matter, because he has friends.

Rachel is the same android model as Pris - but in some ways they are different.

Can Deckard "retire" Pris and still love Rachel?

Can electric sheep and electric toads have any kind of life on their own?

The original book has a more complex narrative, more characters and plot threads, but these are simplified or built in to the mise en scene in the film.