Here is a quick run down of my own little library on the subject:
An account of the German preparations and the British counter-measures
Published in 1957 and with the author being the older brother of the creator of James Bond this book is quite different to some of the later ones on this list. Fleming is quite realistic regarding the flaws in the German plans and presents a clear and balanced view of the opposing plans. It presents a purely factual account and is one of my favourites.
Invasion of England 1940
The planning of Operation Sealion
This account, translated from the original German, provides one of the most detailed explorations of the German plans. It is by far the most useful book here from a gaming perspective as it gives details of the invasion fleet, landing plans and units proposed. Highly recommended.
The Alternate History of the German Invasion of England, July 1940
Macksey's book is essentially a fictional history of the invasion. He decided to base his story on a July 1940 invasion which makes the British defence somewhat weaker, coming not long after Dunkirk but makes the invasion less plausible historically as the Germans simply weren't ready. Of course as a "what if" you can make whatever assumptions you need to make the story work and, had the Germans been serious about an invasion and not been surprised by the speed of their own advance, you can make a case for this approach. It is an interesting read.
If Britain Had Fallen
The real Nazi occupation plans
Based on a BBC television film of the same name, this book is similar to Macksey's in that it is a fictionalised account of the invasion. In this case it takes the more usual September 1940 date for the invasion. Another interesting read.
The Nazi Invasion Plan for Britain
SS General Walter Schellenberg
The introduction to the book by John Erickson puts the remainder into an interesting context. The balance of the book is actually a translation of a handbook, for which Schellenberg claimed a large part of the credit, developed by the Germans in preparation for an invasion. This book is interesting for the glimpse it provides into the German view of Britain at the time; however, it is not really "The Nazi Invasion Plan" as the sub-title suggests. Interesting for background.
Operation Sea Lion
The German plan to invade Britain, 1940
Another translation from German this book has a somewhat wider scope than some of the others in that it covers a much greater portion of the early part of the war in the lead up to Operation Sealion. As such it does provide the appropriate context for the invasion and approaches it from the German perspective; however, it does not go into the level of detail of some of the other books here.
The explosive truth about the Battle of Britain
The "explosive truth" of the sub-title isn't really terribly explosive I'm afraid. It is simply that, whilst important, the Battle of Britain wasn't the only reason the Germans didn't actually invade. Put simply the Royal Navy played a significant part. Even with air superiority or even supremacy the invasion force would still have had to cross the Channel and the Royal Navy was much larger than the Kriegsmarine. Robinson makes his case well, even if it is a rather easy case to make, and tries not to play down the part played by the RAF.
Operation Sealion 1940
Martin Marix Evans
Unfortunately I can't really say much about this one as its still on my reading list. I'm hoping to get on to it after I've finished the three books I'm currently reading. In which case I'll post my thoughts then.