Well, I succumbed to my nerdiness and bought World of Warcraft: Chronicles. I'm a huge sucker for fantasy world-building (hm, I should get World of Ice and Fire next, I guess,) and one of the biggest draws I've had to keep me playing WoW for nearly ten years (well, with gaps like the past couple months when there's not much in the way of content) is their world.
Chronicles lays things down explicitly, though there remain a few mysteries, which I'll include below the cut along with the cool reveals. There's a lot of stuff in here that will tie into Legion - which makes sense, given that I'm sure their creative team has recently been generating a lot of the lore surrounding the Broken Isles, but there's also some good content related to Mists of Pandaria.
As a note: I started writing this and it was getting absurdly long, so I'm starting over and limiting my tidbits to one or two sentences.
So because this is pretty much all spoilers, for the book and the game-world, I'll put it all behind the following cut.
First off: The Titans are all dead. Sargeras killed them. (Ok, so there's one Titan left.)
The Old Gods are basically a tool used by the Void Lords, who are probably the biggest bads of the Warcraft universe.
Titans begin as "World Souls" within some small proportion of planets.
Azeroth is such a nascent Titan, and if it ever emerges, it will be the most powerful Titan ever.
Sargeras was the first Titan to discover the Old Gods. He killed the planet he found them on.
When the other Titans discovered Azeroth, they created the Titanforged to fight them, because the Titans were far too large to actually descend to the planet.
Aman'thul personally killed Y'Shaarj, plucking it out of the planet's crust.
Y'Shaarj was torn apart in the process, but the process left the World-Soul wounded. The blood is the Well of Eternity.
The Titan Keepers are the most senior and powerful Titan creations on Azeroth.
Sargeras found the demons too incompetent on the battlefield in the fight against the Titans, despite their victory, and sought out the most intelligent and well-organized race in the cosmos to corrupt and use as commanders. He chose the Eredar.
The Keepers are: Odyn, Loken, Thorim, Hodir, Freya, Tyr, Archaedas, and Ra (aka Ra-Den.)
When the Titans died, Golganneth sent their souls to the Keepers, but the Keepers couldn't quite understand everything that they had been given.
Ra was the only one who understood that the Titans were dead, and he succumbed to despair and despondency.
The waters of the Vale of Eternal Blossoms are what is left of Aman'thul's soul.
Galakrond was basically the first undead thing on Azeroth, after he started cannibalizing dead proto-dragons (he also ate living ones.)
The proto-dragons were descended from elementals.
Tyr convinced the other Keepers (except Odyn, who was the Prime Designate) to empower the proto-dragons as Aspects and create the dragon race to help them.
Odyn objected to empowering non-Titanic beasts, and created the Halls of Valor as a reward/recruiting tool for the Vrykul.
Odyn forced his closest ally/adopted daughter, the vrykul sorceress Helya, to transform into the first Val'kyr and take the souls of valorous dead vrykul from the Shadowlands to be reborn as his Valarjar.
When the Titans died, Loken was hit the hardest (at least among those in Ulduar.) He took comfort in his brother Thorim's vrykul wife Sif and began an affair.
Loken became prey to Yogg-Saron's manipulations. When Sif tried to break off their affair, Loken killed her accidentally.
Much of the trouble in the world is because of Loken's death-spiral of bad decisions.
Due to his desire to cover up what he did, and the suggestions of Yogg-Saron, Loken introduced the Curse of Flesh into the Forge of Wills, murdered Mimiron (whose Mechagnomes built him a new body,) hypocritically chastised Thorim for starting a war against the Ice Giants (based on how Loken had framed their king for Sif's death) and thus drove him to seclusion outside of Ulduar.
Tyr and Archaedas uncovered Loken's misdeeds, so Loken turned to Yogg-Saron for aid and sent two C'thraxxi (faceless generals like Vezax) after them.
Tyr fought one of the C'thraxxi to let Archaedas and his other allies escape. He died killing the thing and the location was called Tirisfal Glades in his honor. The huge Silver Hand he got after his original was bitten off by Galakrond marked his resting place.
The Night Elves were originally Dark Trolls who were far more reclusive and peaceful than the other tribes. Settling near the Well of Eternity transformed them.
The Yaungol were the first of the bovine humanoids. Some were transformed into Tauren in basically the exact same way the Night Elves came to be, though they didn't settle there.
The Mogu were infected by the Curse of Flesh because Loken wanted to check in on Ra. The Curse started their decline into barbarity.
Lei Shen was the scion of a beaten clan when he went off on his own to find out what happened to Ra.
Lei Shen found Ra and learned about the fall of the Titans. Rather than despair, Lei Shen decided that he would become the Titans' successor, and became a brutal dictator to achieve that goal.
Lei Shen decided to recruit all the Titanforged races to serve essentially as the upper class of his society (because it also included enslaved "lesser" races.)
He allied with the Zandalari and went to Uldum to recruit the Tol'vir by force.
The Tol'vir altered the Forge of Origination to a much more limited range and activated it while most of their people hid within the Halls of Origination. This is how Lei Shen and most of the Zandalari leadership were killed. It's also why Uldum is a desert instead of a jungle.
Oh, and thank Fandral Staghelm for the Emerald Nightmare. When Saronite started popping up everywhere, he planted new World Trees to try to soak it up. These became the great trees where the Dragons of Nightmare used to be as well as Vordrassil in Grizzly Hills. Vordrassil was the one that really opened the Dream up to Yogg-Saron and the other Old Gods.
Jumping ahead, because a lot of the stuff in between we already know.
The humans seem to be decedents of the Dragonflayer Vrykul Clan, while most of the dormant ones are of the particularly-vicious Winterskorn Clan. In fact, it appears that the humans might even pre-date the Night Elves, as the Dark Trolls were only transformed by the Well of Eternity about 15,000 years ago, which seems to be after the reign of King Ymiron (who was the one who declared that the "malformed" children should be killed.)
Humans have been around longer than Dwarves. Humans were around in Tirisfal when the Highborne who would become the High Elves landed there.
Actually, Dwarves are one of the youngest races, as they only emerged 2,500 years ago, at which point the Arathi Empire had already been around for three hundred years.
The Dwarves were awakened by the last Mechagnome in the Eastern Kingdoms to succumb to the curse of flesh. The Dwarves had gone into stasis in Uldaman when they began to suffer the symptoms, while the Mechagnomes volunteered to maintain the facility.
Humans practiced shamanism and druidism (and animism, but that's not a class,) until some of their spiritual leaders communed with crystalline angelic creatures. So yep, the human Light religion is perfectly compatible with the Draenei one, as they both get their power from the Naaru.
STUFF WE DON'T FIND OUT:
Anything whatsoever about Draenor or the Orcs (I'd expect that's going to be in volume II.)
What Elune is. The Night Elves/Dark Trolls believe in her as a goddess who lives within the Well of Eternity during the day. Is Elune really the Titan Azeroth? Or does she not really exist? Cenarius is only believed by the Night Elves to be Elune's son - we don't know for certain.
Where the non-Titanic, non-Old God races come from. It seems that Trolls, Furblogs (who have an ancestor-race that's mentioned once as a fierce enemy of the Vrykul,) Pandaren, Yaungol (and their off-shoots, the Tauren and Taunka,) etc. must have just evolved naturally. We do know that evolution goes way faster on Azeroth thanks to Freya's petri-dish zones of Un'goro Crater, Sholazar Basin, and Vale of Eternal Blossoms.
We learn a tiny bit about Argus, but mostly that Velen, Archimonde, and Kil'jaeden were all awesome leaders before Sargeras came.
CHRONICLING THAT CHRONICLE:
Honestly, there's a ton of stuff I'm leaving out or simply not thinking of. If you know your WoW lore, there will be stretches of the book that are somewhat old news, but there are definitely some big reveals and some cool filling-in of details. Like the fact that Lordaeron was named in honor of one of Thoradin's top generals, Lordain. The biggest revelations are definitely in the early part, but I found stuff like the story of Loken's betrayal pretty interesting.