This is a first rate book that covers functional programming in f sharp. It is straight to the point and does not waste any words (unlike other mammoth volumes you sometimes get on popular .NET languages).
It is a more academic book than others in that the many examples and exercises are the style you would get on a university course, however I believe this is better than the hypothetical examples in other types of books.
I am having to go through each chapter twice, once to read it and again to help solve the exercises. Not everything is covered in f sharp, but what isn't is readily available on line.
The reason for the four stars is more to do with the book binding than the contents, the cover became curled after a short while and it is difficult to lay the book flat to input the examples.