It was published in 1996, written by Paul Chafe, and is published and distributed separately, telling a story that is separate but with references to events mentioned or happened in the game.
The protagonist in the book is Lewis Tyrell, formerly an infantry Captain who led a small unit of the 'Pathfinders', once a United Nations special operations organization commissioned to steal prototype of the Hype serum from an Alliance base in the Tehachapi mountains -- but after the Secession War they were disbanded, since the United Nations lost the war after the player's actions in the game. From this perspective, the book details Tyrell's extensive postwar adventures. These begin after he is treated like an incorrigible criminal, and sent to serve out a life sentence in a lunar prison located at Mare Stellatis and his attempts to escape.
Lexington, the ship where the game takes place, makes a cameo. The protagonist will eventually visit Persephone and the time gate seen in the game, and names such as 60 Ophiuchi, Deneb Kaitos and the Erebus base known from the game, are also mentioned. However the Alliance (who are now the government) is now painted in a poor light, showing somehow totalitarianism and cruelty.
The book and the game both refer retrospectively to some dates and sequences of events that (when they cross) are internally consistent, make sense and fit together properly. The book is providing even more detail than the game about the history, society and politics of the universe. The totality of this available information is what makes it possible to have an adequate summary of the story and background of the Mission Critical universe.