The Complete Book of Villains is a mostly system-agnostic guide to creating antagonists and plots for roleplaying games. This mostly takes the form of high-level advice about things like character motivation and power vacuums. It's decent, as far as it goes. If it has a fault, it's that it will make you neither a great writer nor a hack. It's really more of a reminder that you can apply the stuff you learned in high school literature class to your D&D games.
Many of the book's individual points are illustrated with pop culture references of varying relevance. It's probably okay to mention that Frankenstein's monster was as much a victim as he was a villain, but citing Curley's wife from Of Mice and Men as an example of a villain motivated by lust to cause trouble is . . . not a good reading of what was going on in that novel. There are likely a lot of C- English papers this book has to answer for.
As a group, I'd say that these reference probably obscure more than they reveal. Sometimes one will illuminate a truth about creating a memorable villain, but mostly they just take me out of the D&D mindset.
Luckily, a much greater wordcount is given to original NPCs and organizations. These still only serve as examples of the book's abstract advice, but since the same characters are used from chapter to chapter, you get a real sense of how The Complete Book of Villains' step-by-step villain creation process is meant to work.
Being used as character-building examples winds up compromising the sample characters' effectiveness as villains, so much so that I doubt anyone from this book ever showed up in a reader's real game, but they do work for the purpose they were intended for. I don't buy into the idea that Dog-Eater, the warlord, eats dogs because it's part of his nightly routine, and the bit about him secretly wanting to be a priest despite his violent anticlericism strikes me as the sort of thing that only maybe makes sense in a world where priests get magical powers.
I did, however, find it hilarious that when talking about the Corn Kings (a sample villainous organization), the book was just one or two fantasy cliches away from simply describing capitalism. Yes, they perform human sacrifices to a dark god in order to bless the land's fertility. And yes, their political opponents have a convenient way of turning up dead, only to be reanimated as zombie farm workers, but their whole villainous plan is to get rich selling grains by making the peasants do all the work and then keeping the bulk of the profits for themselves.
The most AD&D thing about this book is the continuing blight of the alignment system. I'm not really sure having a lawful good "villain" makes any sense. They may go on a fanatical crusade, but if they're truly lawful and good, wouldn't they only crusade under justifiable circumstances? Or maybe, if they're crusading in a villainous way, they're either violating the law or being evil. Either way, it's kind of a nonsense section. The whole point of having an alignment system in the first place is to separate heroes from villains.
Also, "True Neutral" doesn't make a damned bit of sense under the best of circumstances, and The Book of Villains may well be its ultimate nadir. There's a wizard who wants to "restore the balance of good and evil" . . . by capturing humans and orcs and forcing them to breed, in the hopes that the resulting surge of half orcs will bridge the gap between the races. Aside from the obvious flaw - any plan that rapey is going to be pure evil - it doesn't make a damned bit of sense why anyone would want that.
I can't say this book has a lot of longevity for me. I'll probably never bother reading it again. I've got better sources of DMing advice and there's not quite enough fun here to make the duplication worthwhile. But it wasn't a totally useless book. Had I read it 25 years ago, I might have managed to learn something.
UKSS Contribution: "Dog Eater" really is a great warlord name. I don't care for much else about the character. His cause is tedious (kill all clerics) and his motives are absurd (because he wants to be a cleric, but can't), but there's a certain over-the-top villainy to eating people's pets as a show of dominance. Also, I need someone to wear the coin armor, and this just feels like it fits.