The full title is X-Men: Future History - Messiah War Sourcebook. It's a "Files" type book, narrated by Cable regarding players in the Messiah War, including a section on himself. The section about Cable's weapons is pretty good, fun to read, especially the last page, which has entries for "Pouches", a little Liefeld joke (he loved to draw little pouches all over his characters' costumes) and "Whatever is Lying Around", very clever. Speaking of Liefeld, be prepared for a lot of 90s artwork in this issue...

Northstar appears very tiny in the entry for X-Men, with an illustration reprinted from the Bachalo wraparound cover for X-Men #200.

Love,
rplass