Basically, a book for players of Elders in Vampire: the Requiem, with rules which make them playable and challenging.
Every aspect is covered, from the mechanical aspects of generating someone who's got a few centuries of unlife under their belt, to unique character generation rules.
The book describes "new" Clans, but these will be familiar to you if you have other supplements such as VII, the Mekhet Clan Book and Requiem for Rome, namely the Akhud, the Hollow Mekhet and even the Julii. These are Second Edition V:tR mechanics for the originals, superseding their original writeups (and providing writeups for some for the first time). Similarly, the Covenants are brought under the spotlight, including one new Covenant purely for Elders, not seen or described before.
A chapter is devoted to the relationship between Elders and time and history; how the passage of time changes them, and how they touch history. There are tie ins with the Dark Eras Chronicles, because this book describes playing those Elders in various historical eras.
What's it like to portray a being with Intelligence or Manipulation 7, or Wits 6, or Dexterity 8? This book looks into that, too, along with what it's like to play beings with skill dots above 5. You may need Chronicles of Darkness: Dark Eras for a look at older pre-modern skills, though there is a summary here.
What else? New powers - Prestantia is back, making it fun to be an Akhud. Elder Devotions. New Cruac and Theban Sorcery Rites, for Elders. New Coils and Scales of the Dragon. New Merits.
A look at unlife, torpor, retaining your Humanity when even the definition of Humanity keeps changing, companionship, relations with foes and allies, new Banes, Elder Banes, Touchstones for Elders, ghouls, new monstrous enemies to fear, crossovers with mages, changelings and Primordial Beasts ...
There's a LOT of new information in this book, all of it geared towards Elders, aimed at creating Requiem Chronicles as far above the regular game in terms of scope as the regular game is above the mortals game from the Chronicles of Darkness blue book.
I'd been looking forward to a really good new book for Second Edition Requiem. Until something even better comes along, this is the first one that really cuts the thirst. As it were.