The Book of the Empress is a sourcebook that tackles a cosmic level antagonist in the Champions Universe: Istvatha V'han.
What's intriguing about it is how the book not only adds layers to the character of Istvatha V'han, but also how she relates to two other well-known cosmic level antagonists in the Champions Universe, and just how complicated invading dimensions and conquering them can be.
The history of her rise to power is not entirely one-note, but the complexities really arise when she begins to encounter other expansionist players in the multiverse with designs on what she claims as her territory (Champions Earth). There are enough interesting wrinkles to that history and the workings of the multiverse to overcome one of the problems of many super-heroic settings not based on ongoing comics series: insufficient twists and turns and a stunningly small amount of major players. In the Book of the Empress, the field that she plays on is complex, peopled with many varied types of opposition, and the history has the feel of a rich comic book universe.
The Empress of a Billion Dimensions is statted out, but has variant versions for other genres, should you choose to limit her magnificence.
Her minions are statted out as well, and a rich mix they are: general forces, several select races, an elite force, and the equivalent of an Imperial Battalion populated with heroes and villains from alternate universes that will be familiar to the heroes of your campaign.
There's also a good explanation of the Champions Multiverse (which is based on the Sephirothic Tree of Life), notes on Imperial Technology, Vehicles, and a guided tour of the Empress's Empire -- government, domains, and how all them are gearing up for the invasion of Earth.
Most useful: the in-depth look into the personality and plans of Istvatha V'han. A must for GMs trying portray a fiercely competent, accomplished, and powerful individual who may, at some point, interact with the PCs.
For someone looking to do an ultra-mega-crossover in a super-heroic campaign, this is the book for you.
(review also appears on http://armchairgamer.blogspot.com/)