Two I highly recommend:
WE SHALL FIGHT ON THE BEACHES: DEFYING NAPOLEON AND HITLER 1805 and 1940 by Brian Lavery
Compares and contrasts the two most dangerous invasion periods in recent history. Includes a map of Coastal Command flights during this period and much discussion of Ironside's fortifications.
OPERATION SEA LION: THE FAILED NAZI INVASION THAT TURNED THE TIDE OF WAR by Leo McKinstry
Much in-depth discussion of Sea Lion. Includes such topics as the political situation, the arrest of Mosley and other pro-fascist leaders, finances and the shipping of gold to Canada, why Ironside was replaced by Brooke, public opinion, anti-invasion measures, the Home Guard and much more. I cannot recommend this book enough if you really want to learn the myths and truth about what chance Sea Lion had.
And the title? Per McKinstry the ONLY chance Hitler had of winning was to take England out. And ONLY an invasion would do that - not the Luftwaffe, not the U-Boats, not threats. And since Sea Lion was cancelled there was no hope of knocking England out of the war...
Hope this helps...