In the future

Pandora's Book:  Antagonists and encounters.  The first chapter goes into a lot of detail about Flux, Pandorans, and Centimani; it includes a detailed guide to the Sublimati, and ideas for non-evil Centimani.  The second chapter is about the Qashmalim, and includes a third "arch" choir, which is above the Elpidim and Lilithim; it also includes six options as to the nature of the Principle.  The third chapter is about Firestorms.  The scenario is set in New Orleans.

Strange Alchemies:  Player's guide, more or less.  The first chapter expands the Lineages, including new Bestowments.  The second chapter expands the Refinements, including new Transmutations.  The third chapter comprises a number of essays by different writers about various aspects of the game.  The scenario is set in Boston.

Magnum Opus:  GM's guide, more or less.  The first chapter is about weird possibilities:  constructs (Prometheans made from materials other than flesh), extemporae (Prometheans created through happenstance rather than deliberate action), Prometheans created from dreams, a myth of the Pyros as a sentient fallen entity, a myth of a wandering Promethean "saint," and more.  The second chapter comprises four new Refinements:  Bronze, who focus on helping and protecting their throng-mates; Cobalt, who study impurity, their own and that of others; Silver, who study the other supernatural mysteries of the world; and Lead, who study the baseline Promethean condition.  The third chapter contains advice on running Going to the Wastes, and other highly personal aspects of the game.  The scenario is set around Denver.

Saturnine Night:  Genre expansion.  The first chapter is about new Lineages, including those made by supernatural demiurges, and those very unfortunate demiurges who blow it and make Pandorans.  The second chapter is about the Unfleshed, which are robots or AI's rendered in similar terms to those of Prometheans.  The third chapter is about the Nuclear Prometheans, which have a number of differences from the default kind.  The scenario is set in Detroit, and includes general information on the city before the action kicks off.

In my opinion, Pandora's Book and Magnum Opus are stronger books than Strange Alchemies and Saturnine Night, but obviously the best one to get will depend on what you want more information on.