What I loved: The book sparks imagination, and the illustrations are really detailed. Some of them are really interesting to view, such as the title image of if fish were leaves or if zebras had stars and stripes. The book really encourages children to imagine the impossible.
What left me wanting more: Some of the pictures are a little horrific, and may be a bit much for some people. For instance, the image of toes as teeth and of mice as hair appear as kind of nightmare scenarios for some people. This may be fine for a lot of audiences, but it is something to be cautious about when diving into.
Final verdict: IF... is a book about imagination and empowers children to think outside the box. Some caution would be encouraged for sensitive readers.