Haskell is a very mathematical programming language. It is typically learned by mathematically inclined people who have big brains. That makes the subject matter frightening to many people. It triggers their "math anxiety".

The title is deliberately silly so as not to make people feel anxious. Haskell books are typically pedantic and formal. The informal, grammatically incorrect, silly style of the title is kind of promise to the reader that the book won't be overly pedantic and formal.

I think also that the author, who is Slovenian, may be making fun of his own English skills. (The English inside the book however is very good.)

In better English, the title might be something like, "Learn Haskell for great benefit".