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."
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.