An official Wizards of the Coast supplement about monster creation and use.
Not pre-made creatures and ready level and statted things but something like the Monster's Handbook or A Magical Society:Beast Builder on creating new creatures from the ground up and tweaking existing ones. And it needs to have some mathematical formula for breaking down a CR.
It can't be considered a nice little add on like the Races book either. The book must be a mandatory Dungeons and Dragons supplement along with the Big 3.
Ah, a gamer can dream, can't he!