Came across a book where it says, air frame cruise drag decreases with an increase in higher altitudes.
Not true in general, e.g. when climbing at constant CAS. However, when climbing at constant Mach close to Mmo it is true because your speed is getting closer to minimum drag (green dot) speed. If you continue at constant Mach below 'green dot', then your drag increases again (assuming you don't run out of thrust or hit buffet).