Demons have been around in D&D since the beginning, of course (specifically since 1976's Eldritch Wizardry.) In fact, they've been so prevalent that the game maybe more fairly should have called Dungeons & Demons instead of Dungeons & Dragons, since they're much more likely to appear than dragons. As well as a detailed appearance in the first Monster Manual, they also have a long and storied history in some of the most iconic adventure modules the game ever produced, which is arguably a much more lasting influence than being in a list of monsters anyway. One reason for the demons' memorability is that they are a combination of rank and file demons and big, important personalities in the form of the demon lords. The demon lords origins are a bit of a mixed bag; some are drawn from mythological or Judeo-Christian demonological sources (Orcus, Demogorgon, Dagon, Pazuzu, etc.) while others were invented whole cloth for the D&D game, following in the tradition of Clark Ashton Smith, H. P. Lovecraft, Michael Moorcock, and other writers with whom Gygax or the other developers were familiar (Fraz-Urb'luu, Zuggtmoy, Graz'zt, Llolth, etc.)
The demons and their lords and their homeland, the Abyss, got an expanded treatment in Planescape, especially in Planes of Chaos and Faces of Evil: The Fiends. They also were present from an early stage in the 3rd edition of D&D, although for this edition, we'd have to wait quite a long time for demon lords to make an official appearance. The demand for more fiends was evident early on in 3rd edition too; Green Ronin's Legions of Hell was their best selling product for many years (according to the introduction of Book of Fiends anyway). Erik Mona himself penned the demonic counterpart to Legions of Hell: Armies of the Abyss. This really established him as a guy who knew his demons and knew what to do with them to make them really quite cool. Armies of the Abyss didn't include stats for demon lords, but it included a lot of them as patrons; more like gods that could be worshipped by evil characters than monsters to be fought by good ones. Mona really dug into demonological folklore here, digging up lots of characters from Judeo-Christian tradition and remaking them in the image of D&D. He also introduced the concept of the qlippoth (again from Judaic tradition, specifically the Kabbalah), primordial demonic entities that predated the demons proper, and still lurked, nursing their hatred, in dark corners of the Abyss. He carefully sidestepped some of the D&D intellectual property; Graz'zt, Yeenoghu, Juiblex and Llolth (and others) were mentioned in passing in transparently different names, as are Orcus and Demogorgon; although the names are public domain, the specifics of the D&D versions of those two are not, and Mona clearly wanted to preserve them without conflicting with D&D sources, so he just let them be.
Although not "official" since it was published by Green Ronin under the auspices of the d20 License, I considered Armies of the Abyss, especially when updated and included in the Book of Fiends for 3.5 edition, to be the go-to, last word on demons in D&D for quite some time, when combined with what was in the Monster Manual. Stats in 3e for the demon lords appeared in The Book of Vile Darkness and while not brilliant, they did the trick for the time being (although I still struggled with the concept that some of the demon lords were apparently less powerful than non-individual demons, like balors. Seriously, huh?)
Erik Mona and James Jacobs freelanced for Wizards of the Coast, and along with in-house developer Ed Stark, gave us The Fiendish Codex I: Hordes of the Abyss in 2006, which then became the go-to, last word in 3.5 demons. Not because it was more official than Armies of the Abyss (although it was) but because it was just one of the absolute best books that Wizards of the Coast put out during the 3.5 run. This book nicely combined traditional D&D demonology with a healthy dose of Lovecraftiana (especially in the form of the Black Scrolls of Ahm by author Tulkhet nor Ahm, which closely resembles the Necronomicon by Abdul Alhazred in feel--and the more overtly Lovecraftian obyriths, including Dagon which is very specifically an homage to Lovecraft.) Best of all, it works well with Armies of the Abyss rather than superceding it. The obyriths are the spiritual descendents of the qlippoth; they occupy the same "role" in the cosmology, as well as providing a useful vector for a more overtly Lovecraftian element amongst the demons. Although among the demon princes there is no Demon-sultan Azathoth who's an obyrith, there very well could have been; he'd have fit right in.
After this, James Jacobs went on to write the immensely popular and extremely high quality "Demonomicon of Iggwilv" series of articles for Dragon Magazine that further detailed demon lords and their demesnes. And as it became obvious that Wizards was going to pull the magazines back in-house and make them digital, Paizo released a flurry of stats and articles for as many of the iconic demon lords as they could. Graz'zt was specifically saved for the first in-house issue (although James Jacobs still wrote the article as a freelancer, I noticed), Orcus got missed, but his updated and more powerful stats were provided in one of the last issues of Dungeon Magazine, along with Obox-Ob. Demogorgon was in literally the last (or perhaps second to last, I forget) issue of Dragon, and an incredibly vibrant Wayne Reynolds picture of him graced the last issue of Dungeon Magazine.
When Paizo, therefore, parted ways from Wizards of the Coast and started up the Pathfinder RPG, it was a shoe-in that they'd do demons well, with both James Jacobs and Erik Mona being "big shots" at Paizo. We got a bit of a taste of this in the Second Darkness adventure path; the drow in Golarion are revealed to be demon worshippers, and in the supplemental material for that adventure path, there's a bunch of stuff about including demon lord worship, very similar to Erik Mona's earlier efforts in Armies of the Abyss. In fact, it's so similar that it's almost (but not quite) identical, and certainly is not a big change in any way from that earlier book.
Whew. Enough backstory yet? Well, it's important to understand the final word in this review in context. Because, in a nutshell, my review is that Lords of Chaos is brilliant work, in the same vein as Armies of the Abyss. However, if you already have Armies of the Abyss (plus the Second Darkness adventure path), Lords of Chaos becomes sadly kind of superfluous. The majority of the book is on how to worship demon lords, and who they are. Although there are a few minor changes, this is mostly reorganized and presented anew from Armies of the Abyss with a touch of new material here and there. Like Armies of the Abyss, Lords of Chaos references the qlippoth (not the obyrith, which is a Wizards IP item again, although conceptually they're still exactly the same thing). There aren't any stats for any qlippoth; for that you'll need the coincidentally enough (or not) just released Pathfinder Bestiary 2. In general, this book is even more careful to tip-toe around iconic D&D demon lords than Armies of the Abyss was; there's no transparently renamed Graz'zt or Fraz-Urb'luu or Yeenoghu, for instance (although there is a transparently renamed Juiblex and they go ahead and put Orcus in there. Although they do point out that he's otherwise occupied and not very interested in Golarion at the present. No sign of Demogorgon at all.) Lamashtu gets pride of place as the Queen of Demons, as well as being an actual deity in the pantheon, not just a demigod like the rest of the demon lords.
After being a good half to two-thirds demon lords, though, the rest of the book includes some spells, magic items, a prestige class for demon worshippers, and a ritual that will transform a character into a demon. There's a bit about the cosmology and ancient history of the Great Beyond; the place of the Abyss and demons therein, which is new for Golarion, although not dramatically different from standard D&D takes on those issues over time. There's an interesting discussion on so-called nascent demon lords; individual demons that aren't quite demon lords (and therefore a bit out of scope for Pathfinder characters to face) but are greater than the greatest of non-individualized demons like balors. These are held out as a the perfect long-term antagonists for a long-running Pathfinder campaign. Sadly, no stats exist for any of these guys, but with a small bit of work, pre-existing stats from The Book of Vile Darkness or Fiendish Codex I: Hordes of the Abyss could be adapted to serve here without much of a hitch, I'd guess. There are stats for a few new demons, though, of course.
So, to restate: if you like demons, and love the iconic role they have as villains in D&D, this is a good book, but it's utility is significantly reduced if you already have Armies of the Abyss and some of the other, earlier d20 books on demons, which it thoroughly retreads without adding much that's new. I have them all, so reading this book felt very much like déjà vu to me. And as an unrelated aside, a lot of the art is bizarrely pixelated and looks terrible.