Not sure why but my game has started to CTD when I select new game from the main menu. I can still start a new game from an already loaded game, but not from the main menu. Any idea why this is happening? All that I did was merge a few book mods into a single esp.

If that merge is the only thing that has changed, than that is likely the problem.
Detail which mods you merged and how.

Also...

I restored a backup of my MW directory before I'd made the changes, then used the new merged ESP's and they worked. No idea why they wouldn't on the original.