Bird has written a fine book, the functional analogue to Bentley's fine pieces. If there is one thing that's common to the functional attitude in program design, it's an emphasis on proof and logical consistency. Bird goes into detail and carefully shows why algorithms perform, and what their costs must be in terms of time and space complexity.
One quibble: the reader, to follow the arguments, will want to write the short code selections for himself, to check Bird's arguments; he'll find himself having to define a number of Unicode mathematical operators, like
U-2209, for example. But this isn't hard to do, in Haskell.
Each chapter is well-written, to the point, and closely argued. In showing the beauty of Haskell in a clear way, or showing the beauty of concrete maths in a clear way, Bird has done well.