Chapter one provides basic rules for level progression beyond level 20 and contains a list of epic feats that only epic character can take. The "skills" section shows the superhuman things you can pull off if you succeed on rolls with an extremely high DC (Difficulty Class). It also contains a couple of epic prestige classes.
Chapter two describes the new Epic Spells. Providing the spellcasters with potentially infinite power, an epic spell has first to be researched and invented (which can take a lot of work), and once it is known, casting it requires a (usually exorbitant) Spellcraft DC check. The chapter goes in great detail to explain how to determine this DC; obviously, the more powerful a spell, the higher the DC, but mitigating factors (such as hit point drain) can lower it. Unfortunately, there are clever uses of mitigator/bonus combos that cancel each other's effects out but still net negative points; apply as needed for arbitrarily powerful, super-cheap spells. The chapter also contains a large number of example epic spells, including ones which teleport someone into low-earth orbit or bring a copy of the caster from the future.
Chapters three through five contain tips and advice for the GM, as well as a collection of epic magic items (no surprise here: these are like ordinary magic items, except more powerful, potentially infinitely so) and monsters (starting with the Abominations, unwanted offspring of the gods; the rest of the monsters aren't pushovers either.)
Chapter six, "An Epic Setting", describes a number of example "epic organizations", such as a multiversal assassins' guild or an organization of epic-level bodyguards; describes the extraplanar merchant city of Union, which is basically Sigil, except much friendlier; and contains an example epic-level adventure, "Kerleth's Tower", set mostly in a wizard's tower on the Elemental Plane of Fire.
A supplemental chapter stats the epic-level NPCs of the Forgotten Realms and Greyhawk settings in accordance with the new rules.
Tropes found in the book include:
Ancient Conspiracy: The Regulators. If you believe the rumors, they are older than humanity itself.
Awesome, but Impractical: Many epic spells, if only because their Spellcraft check DCs are so high so as to be impossible for most characters to pull off, unless they have an absurdly high level.
Vengeful Gaze of God. Its Spellcraft check has a Difficulty Class of 419 (probably the highest DC ever to show up in any D&D book), which is pretty much impossible even to a wizard on level 100. At this high a level, the supposedly mega-powerful spell would be little more than a sting to the monstrosities you'd be theoretically facing.
Aid Another Actions, Int bonus items, wishing for stat increases, "turn me into as monster that has a vast int bonus" rituals, Skill bonus items, and dumpster diving for bullshit bonus granting Feats and Pr Cs work wonders.
Becoming the Mask: One shop in Union is run by an efreet named Kharlin, who started it as a front for whatever evil activities he wanted to run. Much like Dr. Evil's organization, however, the shop turned out to be more profitable than anything he could come up with, and now he's fully devoted to expanding its wares and keeping it running.
Beyond the Impossible: The Epic Magic Items can specifically go beyond the normal unbreakable limits for how much enchantment can be put on one item. The random epic magic item tables place no upper limit on how high a weapon or piece of armor's plus can go.
What the epic characters can do, in general. Walking on clouds? Squeezing through a gap smaller than your head? Beheading someone with a single karate chop? Turning your sworn enemy into a fanatical follower with sheer force of persuasion? All this, and more, are allowed as long as your skill is high enough or you have the appropriate feats.
Cloning Blues: Averted for the spell Eidolon, which splits off a small piece of your soul to create a temporary ally. The eidolon has no issues with being a copy (or having a limited duration), as it's still a part of you for all intents and purposes.
Deader than Dead: The assassin guild Garrote can do this to people, usually by destroying their corpse with a special substance. It will also happen to you if a Demilich manages to eat your soul.
Death of a Thousand Cuts: The Crown of Vermin spell. Summons a thousand insects that surround you in a lasting aura. Each insect automatically does 1 point of damage to anyone within the aura, then dies. Pretty useful for auto-killing people. (Unless they have even the tiniest damage reduction of the "impenetrable" type, in which case, they're completely immune.)
Eldritch Abomination: A lot of the monsters look like something Lovecraft would think up; visibly the authors felt that there isn't much else that can challenge you when you're powerful enough to kill elder dragons and demigods.
The Abominations; malformed offspring of deities which desire to destroy all reality.
The pseudonatural creatures. Horrifying, tentacled, soul draining creatures from the Lovecraftian Far Realms, the lesser of which can take on greater demons such as balors. Did I mention they're ridiculously resistant to spells?
Fake Ultimate Mook: Common among epic monsters. Plenty of them can't fly, turning them into sitting ducks for any archer with a Phantom Steed. Others have fairly low ability scores and no defense against damage or drain; a savvy third-level wizard could defeat a Behemoth Gorilla with a single casting of Ray of Idiocy.
Kill the God: The abominations are technically very weak (rank 0) deities, so slaying them counts as this.
Maximum HP Reduction: The Lavawight and Shape of Fire have the blazefire ability which does exactly that.
My Future Self and Me: The Time Duplicate spell is a very limited version of this, as it works only for six seconds, snatching a version of you from six seconds in the future. It's also stated that since this spell "stretches time and probability to its limit", it's impossible to pull off anything more spectacular.
Portal Network: Union, as a planar metropolis, has organized portal commute to all the important planes.
Professional Killer: The glooms, apparently. Whatever they are; they have practically no flavor text whatsoever—all we know is their stats and appearance.
Reinforce Field: Kerleth's tower from the adventure has walls made of two thick layers of obsidian with a thin forcefield in-between.
Serial Escalation: The DM is expliciticly encouraged to do this to the players with ridiculous and unfair challenges on the grounds that the players will have the resources to deal with them. How far can the Dwarven Defender swim through lava?
Time Master: The Abominations known as Phanes. They're unwanted children of deities of time/destiny who look like black clouds of various shapes. They can travel back in time, summon a copy of themselves from the future, summon copies of their opponents from an alternate timeline, freeze themselves or others in time and devour their future (which basically makes their victims grow old at an accelerated rate.)
The Worm That Walks: Dead spellcasters that have become the Hive Mind for an army of worms. Usually it's the evil ones that choose this method of life after death. There's a spell for turning a corpse into one, though it tends to fail.
Undead Child: The Atropal is an undead god fetus. It's also one of the most horrifying things D&D has ever introduced.
Underground Monkey: The Lavawight and the Winterwight have exactly identical stats, except that one of them has the cold subtype and the other one has fire, and their blightfire/blazefire abilities are a little different. Same applies to their usual creators, The Shape of Fire and the Shadow of the Void.
Unwitting Pawn: One of the adventure ideas is about a dragon who tricks the idealistic player characters into starting a gigantic crime-fighting organization. Her sole goal is to find one specific artifact stolen from her long ago.
The Wild Hunt: The fey known as Hoary Hunter comes after its prey on cold, moonlit nights. The only way to get a Hoary Hunter off your track is to kill it and its fellows, evade it nine times, or live the rest of your life somewhere that never, ever experiences the Hunter's preferred conditions. And it will wait for decades if necessary.
Difficult as it might be, your best chance is evading it nine times - if you pull that off, all Hoary Hunters will forever leave you alone.
World of Badass: The city of Union is filled with lots of epic-level people. Even the town guards are very high-level (well, they need to be.)