I answered "maybe". It depends on what kind of "evil" and "mature" topics the author wants to address and how well is the book written.
What I'd like to read about:
- A consistent description of what "evil" means in the context of given setting and what it does not. Modern ethics are completely unfit to most fantasy games, so we need a definition to know what the author is talking about.
- Importance of clear social contract in a game with mature themes. Which topics do we want to address, which we want to avoid? Do we limit how far we're going to go and, if, so, how? Etc.
- How to create characters that are indisputably evil, but have sensible personalities and motivations and are playable in a party. How can an evil character be heroic (even better: heroic and disturbing at the same time)? How characters fall? How they are redeemed?
- How to create villains that are not "evil for evil's sake", but have coherent reasons for their atrocities. How to make evil repulsive, how to make it fascinating?
- Examples of evil societies that are not self-destructive; what shaped them this way, how they develop, what problems they face, what makes them stable. How do they look from inside, how from outside. That would also be a good place to give examples of coherent and sensible worldviews that are definitely evil.
What I don't need to see:
- Evil races, classes, spells, items etc.
- New, "more evil" or sexually themed monsters
- "Evil aesthetics" and other traits of comic-book villainy
- Calling things that are heroic in given genre evil
What I'd like to read about:
- A consistent description of what "evil" means in the context of given setting and what it does not. Modern ethics are completely unfit to most fantasy games, so we need a definition to know what the author is talking about.
- Importance of clear social contract in a game with mature themes. Which topics do we want to address, which we want to avoid? Do we limit how far we're going to go and, if, so, how? Etc.
- How to create characters that are indisputably evil, but have sensible personalities and motivations and are playable in a party. How can an evil character be heroic (even better: heroic and disturbing at the same time)? How characters fall? How they are redeemed?
- How to create villains that are not "evil for evil's sake", but have coherent reasons for their atrocities. How to make evil repulsive, how to make it fascinating?
- Examples of evil societies that are not self-destructive; what shaped them this way, how they develop, what problems they face, what makes them stable. How do they look from inside, how from outside. That would also be a good place to give examples of coherent and sensible worldviews that are definitely evil.
What I don't need to see:
- Evil races, classes, spells, items etc.
- New, "more evil" or sexually themed monsters
- "Evil aesthetics" and other traits of comic-book villainy
- Calling things that are heroic in given genre evil