For a start, 'Angels of Caliban' could be a good choice. It has elements of mystery and the connection to chaos is gradually revealed. It's a bit different from the more straightforward chaos - centered novels but still gives you a taste of how chaos is intertwined with the different factions in the 40k universe. 'The Damnation of Pythos' can also be a starting point as it throws you right into a world being consumed by chaos, which can be really exciting.
I'd recommend beginning with 'Gaunt's Ghosts' series. It offers a good mix of action and character development, giving you a solid introduction to the Warhammer 40k lore. Also, it's relatively accessible for newcomers.
One of the great ones is 'Horus Rising' which sets the stage for much of the chaos in the Warhammer 40k universe. It gives a deep look into the fall of Horus and the start of the chaos within the Imperium. Another is 'A Thousand Sons', which delves into the story of the Thousand Sons legion and their fall to chaos. It has great character development and an interesting take on the chaos powers at play. And 'The First Heretic' is also a top pick as it shows how the seeds of heresy were sown among the Legions, leading to chaos spreading throughout the galaxy.
Fanfiction in the Warhammer 40k Chaos Gods realm can be really diverse. Some stories could be about the power struggles between the different Chaos Gods. For example, Tzeentch, the Changer of Ways, might be plotting against Nurgle, the God of Decay. Writers might also create new characters who are either lured into the service of the Chaos Gods or are trying to resist their insidious pull.
I'd recommend 'Horus Rising' as a great one to start with. It's the first book in the Horus Heresy series which is a huge and important part of the Warhammer 40k lore. It gives you a good introduction to the Space Marines, the Imperium, and the main characters like Horus. You get to see the events that start the whole downward spiral for the Imperium.
If you're new, 'Gotrek & Felix: The First Omnibus' is a great start. It has an easy - to - follow story and introduces you to the Warhammer fantasy world in an exciting way through the adventures of its two main characters.
You could start with 'Do Androids Dream of Electric Sheep?'. It's a classic and gives a great taste of his unique style and imagination.
First, decide if you're more interested in a particular faction, like the Orks or the Eldar. Then look for novels that focus on that faction. For instance, if you like the Space Marines, start with a novel like 'Ragnar Blackmane'. It follows the story of a young Space Marine. Also, consider borrowing some novels from the library first to see if you really like the writing style and the overall atmosphere of the Warhammer 40K novels before buying them.
The recommended order to read Warhammer 40k novels can vary depending on personal preferences. But generally, starting with the core series like 'Horus Heresy' can provide a solid foundation.
Isha can be seen as a catalyst in many fanfictions. Her presence in the chaos realm stirs things up. She might be the key to a certain prophecy or the cause of a power struggle among the chaos factions. In some stories, her very existence in the chaos is what drives the plot forward as different characters try to either use her or protect her from the chaos forces.
One good one could be 'The Redemption of Isha in Chaos'. It explores Isha's journey within the chaos realm in an interesting way, with great character development.