There are two possible reviews of The End of Harry Potter?. I've chosen to write the one that contains spoilers for 1-6 but not for 7, though
It'd be tricky to avoid all spoilers, though:
"When you're trying to look at every detail of the ingenious way in which J. K. Rowling leads up to letting a particular cat out of the bag, it's difficult to conceal the identity of the cat. Though usually, I think it's safe to reveal at this point, the cat is not Mrs Norris." (p.3)
Langford's evidently an admirer of Rowling's craft, her ability to construct a thrilling and plotty novel (though he also draws our attention to improvements in style and structure over the first six books.) There are chapters on 'Guns on the Wall' (items casually introduced which turn out to have immense significance); 'Smoke and Mirrors' (misdirection); on names, foreshadowing, the genetics of the wizarding world, and on Unfinished Business. There's also an affectionately mocking chapter on slip-ups (though he doesn't point out that it took JKR three books to learn how to spell 'minuscule'). And there's a fascinating section on the subjects that JKR has avoided discussion of: Lily's profession, Sirius' motorbike, cats, scars ...
All of which lead up to the meat of the book, Langford's predictions of how the series might end. After a wickedly funny LOTR pastiche (in which Harry and Neville lug the last Horcrux towards Voldemordor, followed by Kreacher who "makes little gulping noises in his throat that sound like gollum, gollum. Harry has not read enough twentieth-century fantasy to be worried by this") and some cutting-room floor possibilities (the Star Wars climax, the Gone With the Wind version, and Some Like It Hogwarts
There is also a section on likely fates of major characters (in which I gaze sadly at the word 'surely'). Less accurate speculation here, but still convincing.
And the last section, 'The Ultimate Secret', is spot on: "J K Rowling is still going to surprise us all."
Impressive detective work, the usual Langford wit and evident enjoyment of the HP books: an interesting read.