Comment by Kototama

It's hard because there are so many concepts to understand. After reading one Python book you can write solid programs in Python. Not so in Haskell, you would need to understand also the extensions of the language which are popular and understand the best practices (what to use to compose I/O and in which context for example), on top of all the basics. That and understand how to work with complex types in libraries: that require time. That would be too much for one book.