My 2.5 year-old daughter, who often takes a while to warm to new books, LOVED this at first sight. One or more of the animals is 'blanked out' on every alternate page, so the child has to figure out who is missing. The pages in between ask different questions: "who's backwards?", "who's angry?", etc. It works on different levels - it can help a child to recognise different animals; improves memory (sometimes I can see my daughter trying to remember which animal is hiding from the last time we looked at the book together); and helps children to recognise and understand different feelings - one page asks "who's crying?".
The only reason I haven't given this 5 stars is the slightly confusing use of colour - the pig is blue, and the orange and black zebra looks alarmingly like a tiger! This certainly hasn't hampered our family's enjoyment of the book though.