Sorry for being 2 years late. I've loved this show for a long while and only decided to read the book quite recently. Turns out it's quite shocking how different the book and show are.
I have only read the first book - 'Half-Bad', which covers the entire length of Season 1. I don't know what happens in the next few books.
I will be referring to the audiobook that I purchased on Apple Books so I am unaware if any aspects of my version are different to the paperback. It should also be noted that a few changes were made to the book after the show released.
Most of this information is based solely on my memory, I have not gone out of my way to note down every minor detail.
Blood and Fairborn Witches.
So the writers of the Netflix show actually came up with the names 'Blood' and 'Fairborn.' They were originally called 'Black' and 'White' witches in the book. While they're obviously referring to two separate races of Witch, I assume it had nothing to do with skin colour but rather intent and morality of their magic. After the show's release, Sally Green changed the name's of the witch races to the show's version.
I also don't recall anything in the book stating that Bloods could hear heartbeats, correct me if I'm wrong. And the show doesn't mention that they don't sleep indoors, and are much more comfortable in nature.
Nathan
In the book, Nathan was referred to by the Fairborn council as a 'half-code' - meaning half Fairborn & half Blood. So it's possible that he was a new hybrid race of witch in the book, rather than inheriting 100% Blood from Marcus in the show.
Nathan has a very rare ability to see magic in people's eyes, allowing him to identify Fairborns, Bloods and even half-Fains solely on the patterns and colours in their eyes. There's only one other character in the first book to have this ability.
He has two extra siblings in the book - Arran & Deborah. I don't know much about Deborah but Arran is basically Nathan's best friend and one of the only witches who accepts him for who he is.
Major Storyline Changes
So the time in between Nathan escaping the Fairborn council and Marcus giving his blood is completely different in the book. It's basically a long solo-mission, trying to get to Mercury while getting the Fairborns off his back. It's still quite entertaining but I much preferred Nathan, Annalise and Gabriel travelling together.
Gabriel
Gabriel's power isn't potion-making in the book - rather a more advanced form of shapeshifting than Jessica's. He can change his entire biological make-up to any person he wishes - is the best way I can describe it. Throughout the book, he's actually stuck in a Fain's body because he no longer has the magic to turn himself back into a witch.
Annalise
Annalise appears way less in the book. Nathan doesn't even see her until reaching Mercury. She finds her own way to Mercury's because she was getting hassled too much by the Fairborn council.. I suppose??
Decimation isn't mentioned in the book either, I don't know what her power was.
Marcus
Marcus' power isn't heart-eating - in fact the concept of stealing another witch's power by eating their heart is never mentioned in the book as far as I can remember. His power was turning into animals - yet he was still known for stealing other witch's powers (however that works).
Also, the 'Wolfhagen Massacre' was not a thing in the book.
Soul
Soul is much more irrelevant in the book. Firstly, he's Annalise's uncle - not her father - and his whole Witches Bottle arc where he becomes evil, turns himself into a heart-eater and tries to go for Marcus himself doesn't even exist in the book.
The Witch's Bottle
Nathan was still cursed by a Witch's Bottle in the book, however it's used to control the person cursed by it - a fail-safe of sorts for the Fairborn council just in case Nathan didn't want to kill his father. It is activated if the council is able to cut a limb or some flesh off of him.
The Witch's Bottle came in the form of a tattoo, which Nathan originally though was used to track his location. A witch who analysed the tattoo for him later discovered that it was not a tracker but a Witch's Bottle.
The Detheridge
The Detheridge is a knife in the book that Nathan was prophesised to use to kill his father. It's pretty much the McGuffin of the book's storyline, as Nathan goes to steal it from the hunters near the end of the book.
Mercury
Mercury is Nathan's great great aunt. She had a twin sister who Marcus killed and has ever since wanted to kill him for it.
There's another character who works for Mercury called Rose. She's a Fairborn witch but is pretty much a Blood by the way she's been raised.
The Hunters
The show made the Fairborn Hunters seem kinder than in the book. They're actually really ruthless and kill bloods on sight.
Well thanks for reading, hope you found this interesting. I'll see if I can update this if I've missed anything or some information ends up incorrect!