I have this book as well has "How to Prove it." They cover virtually the same material, however I found the prose in "The Haskell Road" to be stilted and sometimes confusing. I got much more out of "How to Prove it."
Interesting; thank you for providing your perspective.
Both books indeed share most of the topics, although from glancing at both TOCs HR seems to add Polynomials and Corecursion.
I'll gladly keep this in mind should I get stuck with HR.
This isn't to say that HR is bad, on the contrary it's quite good. Be prepared to use Google though. How to Prove it is much more self-contained.
It is true that HR covers Polynomials and Corecursion. If I were you, I'd order both and use How to Prove it if you get stuck with The Haskell Road. They're both very inexpensive.