"First introduced in Halo 2, the map is set within the New Mombasa locale."
Headlong is not part of Halo the first, nor is New Mombasa. AFAIK the New Mombasa location was introduced in Halo 2/ODST/Halo 3. Whereas Halo Combat Evolved was on the first Halo. Have no idea about the books but then I wouldn't read a book about the Quake/DOOM back-story either.
Call of Duty is made and released every year along-side the next being pumped out for the next year. Some of the same teams are involved. That does not equate to Black Ops assets being intended Modern Warfare 2 just because they were created during the same time period.
If you have some evidence of these claims I would love to see it.