House of Steel: The Honorverse Companion  is the first of three installations by the collaborative team of David Weber and BuNine. The first half of the book is a novella "I Will Build a House of Steel". It introduces the reign of King Roger, the beginnings of the Manticoran Star Kingdom and the development of the Manticoran navy.
The second half is the compendium which covers technology, specs, history, notable people, key planets and information on the Protectorate of Grayson. It goes into back story of some of the established characters such as: Hamish, Sonja, Janacek and High Ridge. There is also an author question and answer section at the end.
If you are new to the Honorverse this edition doesn't really have any spoilers, but if you want to start with the first book of the series, read On Basilisk Station .
For those of you keeping up with the series, the most recent release (aside from the Honorverse Companion) is Shadow of Freedom .