My copy is the 2000 edition. I ran across the same information in "Tanks of the World, 1939-1945" by the same two authors, which is a 2002 edition. I really don't think the 12" thick applique armor is correct but from photographic evidence and also other sources I don't believe the 114-140mm hull values are correct either. I brought this subject up to illustrate that conflicting evidence is quite common out there.
I'll give you what I have from "Tanks In Detail #6: Medium Tank M4 (76mm & 105mm)" by Terry J Gander:
The M4A2E2 "Jumbo" assault tank was a basic M4A3 with additional hull and turret armour. The hull was 3.94 inches (100mm) thick, and the turret was an all-new box-contoured component with frontal armour of 5.91 inches (150mm) thick. Pictures seem to indicate that the hull armour increase was only to the frontal plates, but the turret received its additional plating all around the turret sides.
Information from this site (http://www.onwar.com/tanks/usa/data/m4a3e2.htm) indicates a similar view. The armour ratings for the front hull are 114-140mm, the front superstructure 102mm, and the turret 152mm on all three facings (front, side, and rear) with a flat-plate mantlet of 178mm.
I'm not sure what edition of Chamberlain & Ellis you have, but my copy is from 1969, and it only states that,
Additional armour was added to all hull surfaces, giving a maximum thickness of 100mm. A new heavy turret was designed with frontal armour of 150mm but the 75mm gun was retained.
Your edition may have a misprint regarding the amount of armour added to the vehicle in the narrative portion.
Artillery always has the Right of Way