I’m afraid you answered your own question when you noticed the variation in identical letters. If ’similar’ is good enough, some of Rob Leuschke’s script fonts have this feel.

- Mike Yanega

This is another example

http://www.amazon.co.uk/gp/reader/0141029331/ref=sib_dp_pt#reader-link

However I still hope that it’s a typo.

And once again you can see that each letter is unique. Each one of the three letter a’s is different, for example. Rob Leuschke’s Kolker Brush shows a lot of similarities to your lettering. Leuschke used to do Hallmark card lettering and he continues doing custom lettering. I’m not saying I think he did the book titles, but someone with a similar brush style did.

- Mike Yanega

Too shame that it’s not a type yet.

It says on the back covers of the UK editions “Calligraphy by Ruth Rowland”. She & Stephen Raw seem to be the usual suspects for hand lettering like this on UK book jackets.