Hmmm well this was clever, and I love Stephen Fry's authorial voice, but somehow it ended up making me sad. Then again I wasn't in the best mood to start with so it may be I would have been more amused if I'd been in a better mood to begin with.
MINOR SPOILERS - Nothing you wouldn't get from reading the blurb.
I liked the premise quite a lot, that preventing the birth of Hitler might not produce a better world after all. Because yeah, Hitler was an evil man but he didn't invent and prosecute WWII all by himself.
But somehow the working out of the premise was a little bit flawed. There was a lot of time spent on the lead up to preventing the birth of Hitler. But once the protagonist finds himself in this new world where Hitler was never born, he just looks up some stuff in the library and says oh I don't like this, and races to get home again.
Which all falls together pretty fast - given that he's stranded in a strange world he sure figures out how to turn it around without much difficulty. It felt a bit uneven. Still, I liked it enough I'll probably read something else by Fry at some point.