I think 'The First Heretic' is very popular. It has a complex plot that involves the fall of the Night Lords legion from their original state. It also has great character development for the main characters within the Night Lords.
One of the notable Night Lords in novels is Talos. He is often a central figure, showing the complex nature of the Night Lords. He has his own moral code, albeit a very twisted one compared to most protagonists.
The Night Lords in novels are often characterized by their nihilistic and sadistic tendencies. They don't just fight for victory; they seem to take pleasure in the suffering of their foes. Their origin stories usually involve some form of dark pact or a fall from grace. Their appearance might include elements like spiked armor and dark symbols. They are also likely to be masters of infiltration, able to slip into enemy lines unnoticed and wreak havoc from within. Their culture within the novels often revolves around a cult - like worship of their own power and the ability to instill fear in others.
I'm not sure specifically as there could be various themes. Maybe they focus on the dark and brutal deeds of the Night Lords legion, their campaigns, or the inner workings of their traitorous society.
One key theme is the nature of loyalty and betrayal. The Night Lords are part of the larger Horus Heresy, which is full of shifting allegiances. They have to decide where their loyalty truly lies, whether with Horus or with the Emperor, or perhaps just with themselves.
There's also 'Soul Hunter' which is part of the Night Lords series. It really shows the brutal and shadowy nature of the Night Lords. The story is filled with action and some really interesting character development.
You can try looking for them in local bookstores that specialize in science - fiction and fantasy novels. Some big chain bookstores usually have a section for Warhammer 40K related books.
Talos is one of the main characters. He's a complex figure, often conflicted between his loyalty to the Night Lords and his own sense of morality. Another is Sevatar, a powerful and feared leader within the legion.