There's a couple of good sources on Operation Sea Lion freely available online. Basil Collier's The Defence of the United Kingdom, originally published in 1957, has been digitised by the good folks at Hyperwar. Coller's work is the British Government's official history of the actions taken to defend the British mainland during the war, and as such, contains several chapters detailing the British anti-invasion preparations and plans. It also has a good chapter on the German plans themselves. Meanwhile, this document, German Plans for the Invasion of England, 1940 Operation "SEALION", produced by the OSS in the aftermath of the war, is a good summary of the German planning process, and the high-level discussions involved in it.

For things you can purchase, Peter Fleming's Operation Sea Lion: An account of the German preparations and the British counter-measures is a good overview. For a detailed look at the German planning, Peter Schenck's Invasion of England 1940: The Planning of Operation Sealion is worth trying to find, despite a few flaws (mainly asserting that Hitler never actually intended to invade). However, it only devotes four pages to the British response. To cover this, you might want to read Bernard Lowry's British Home Defences 1940–45, which covers the ways Britain defended itself against attack - though with all Osprey books, it is relatively light on analysis. Kieser's Operation Sea Lion: The German Plan to Invade Britain, 1940 gives an interesting, but again German-focused perspective, mainly centred on the Luftwaffe's approach to the plan. Leo McKinstry's Operation Sea Lion: How Britain Crushed the German War Machine's Dreams of Invasion in 1940 is a more recent look, and one that focuses more on the British response in detail. However, it has a rather jingoistic tone to it, and a rather sentimental, rose-tinted view of Britain and especially Churchill. It's probably worth a read, but I wouldn't rely on it. One I'd avoid is Forczyk's We March Against England: Operation Sea Lion, 1940-41, which is overly sympathetic to the Germans, and ignores basic facts of geography, as well as the states of the German military and Royal Navy, in order to argue that Sea Lion was not only possible, but likely to succeed had it been carried out.