I came up with this one late last night, and decided to look at several series of books, rather than individual books. Forces of darkness are primarily the provenance of fantasy. Realistic books contain nasty bosses, catty rivals, or heinous ex-boyfriends, but not any real evil, generally. So the books I picked to discuss are the Prydain books by Lloyd Alexander, the Harry Potter books by J.K Rowling, and the His Dark Materials trilogy by Phillip Pullman.
The oldest of these is the Prydain books. Written in the 1970s and 1980s, these are the most like classic fantasy. They tell the story of Taran, an orphan raised by an enchanter whose main duties involve the care and feeding of an oracular pig. In the first book, his charge wanders off and he chases it away from the enchanter's farmstead, far into the woods...and smack into the crown prince of the realm. He is able to prove his worth to this man and assist him on his journey. As Taran matures, he grows into a worthy fighter and adventurer and is aided by the companions he meets in the first book. With the exception of the peripatetic, introspective Taran Wanderer, all of their adventures come in thwarting one man: Arawn Death-Lord, King of Annuvin, who seeks to destroy and conquer Prydain.
The Harry Potter plotline also, of course, is a coming-of-age story. But by the time it was published, readers liked to understand their evil a little better. The Prydain books belong to the protagonists, to Taran and Gwydion and all the friends they meet on their journeys. It's not until the very end of the final book that we come face-to-face with Arawn, and then he's killed almost immediately. In Harry Potter, evil gets a bit more of a backstory. Voldemort can be understood in ordinary psychological terms as a sociopath. Anyone who's ever watched one of those A&E specials on serial killers will recognize Voldemort's background: raised in a cold and stark environment, has an utter lack of empathy for others, sees his fellow humans as tools rather than people, conceals all of this with a smooth charming facade. It's easier to understand how he gained so many followers this way. Dumbledore himself explains that he attracted those with a bent towards cruelty who wanted new outlets as well as the weak who sought protection and glory that they were unable to get on their own.
What remains obscure is Voldemort's point of view. We know only that he disliked "mudbloods" and Muggles. His insistence on racial purity can't help but evoke Hitler to a modern reader, but it's absent the ideology. It's easy for the reader to understand the psychological reasons for supporting Voldemort, but harder to understand the intellectual reasons. The Death Eaters clearly had wonderful group cohesion, but the goal towards which they were working always remained somewhat murky in my mind.
His Dark Materials probably portrays the forces of darkness most realistically. They aren't united, for one thing: sometimes they work together, but they remain distinct entities with their own viewpoints. They're also composed, mostly, of ordinary people. They also believe that they're right, and carry out the most monstrous deeds convinced that it's a means to an end. The gruesome research of severing the bonds between human children and their "daemons" (an external part of themselves) was done to help people, much like experiments on lab rats, except in this case, they had no choice but to use humans, as no other creature has a daemon. The head of these experiments is also the protagonist's mother, and is one scary lady when we first meet her. But, surprisingly, she winds up repenting before the books are over. Lyra's father, on the other hand, seems at first a positive figure, but the pendulum swings several times before his own end.
Lloyd Alexander also plays somewhat with the idea of good and evil being inherent in everyone, particularly in the second book. This is probably the main flaw of the Harry Potter series, though. None of Harry's friends are seriously tempted in any way by the Death Eaters, nor do any of the Slytherins ever express so much as a glimmer of desire to support the Order of the Phoenix. My friend Sophie wondered why they didn't just lock up everyone who was sorted into Slytherin as a precaution and be done with it. The worst Harry ever faces is the transference of Voldemort's soul as a result of the curse that failed, but it never serves as any sort of temptation.
I think the closer definition of the forces of evil is part of a trend twoards greater realism in fantasy. Most of the popular recent fantasy books have been set in the modern era. The protagonists drive cars and watch television. In The Amulet of Samarkand, even the secrecy of magic was gone, so that being a magician was rather like being a plumber or a doctor. It's a fine line, for going too far with that will make the fantasy cease to be compelling. But perhaps modern readers want to see themselves in what they read, rather than getting swept away into another world.