Inspired by this other question, I had a look at the book (published in 1998/2003), and noticed this passage.
When we captured Liège, we discovered a German map factory in full production. We moved a signal corps cartography group into the factory, and they started making maps for the American army. [... eventually printed on the back of German maps] As I folded some of the 1:125,000 color maps, I noticed a lot of blue on the back of two of them. Upon closer examination, I was surprised to see that these were maps of southern England prepared by the Germans for the invasion of England in June 1940. They covered the area from the Thames estuary down through Weymouth and Bournemouth, the ports from which we had embarked. The maps continued farther west and covered all the major potential invasion sites on the English Channel.
When I saw these maps, I remembered when we had first arrived in Codford in September 1943. [... and found a British map which] covered an area south of Codford, which was in Wiltshire, extending along the coast for about fifty to sixty miles. The British camp to which we had been assigned was apparently some type of command headquarters for the defense of this area. The map showed the location of every unit from battalion down through company, platoon, squad, and outpost.
From my limited knowledge of defensive tactics, I could see immediately that this entire area was grossly undermanned; it was covered by only about a thousand soldiers instead of the usual five infantry divisions plus at least one motorized or armored division in reserve. The map showed a battalion area where a division would have been expected, and a company where a full regiment should have been. The defenses gradually diminished to the point where they had to actually cover the beaches. There were not enough outposts to cover all the exits from the beach. It appeared that there were outposts manned by at least an automatic weapon with a couple of men spaced at intervals of a mile or more along the cliffs above the beaches. They apparently used some type of messenger on a bicycle to communicate among these outposts. This British map was dated June 1940 and covered a small portion of the same area of the German map that I had just been issued.
I have often wondered if the German decision to invade England would have been affected if they'd had a copy of this map when they made their German maps in Liège. A single German division could have completely overwhelmed the pitiful British defenses shown on this map.
(Cooper, Death Traps, pp. 129-30)
Is this new information, or has it been known before? Is the generallly accepted evaluation of the proposed invasion as utterly impossible based on the true facts?
Clarification: for the question to be useful, and stay within the scope of the site, let us focus on the map, the British map from the summer of 1940 that cooper says he saw in 1943. Do we know which map this was? Are these maps publically available now? If not, what happened to them? Alternatively, do we know that maps like this never existed and that Cooper must have been hallucinating?