The Odd Egg by Emily Gravett is a definite favourite with Daughter, Husband and I. It tells the story of a group of birds waiting for their eggs to hatch, and duck - who doesn't have an egg of his own.
Personally, I love the quirky illustrations. Daughter finds it incredibly good fun to turn the half-pages which have been cut away to reveal the different eggs hatching. This is a really clever, visual way to show time passing and little fingers find it very easy to turn the board pages. The only criticism I have with it is that she finds them too fun and will keep returning to this page again and again, making it impossible to reach the (incredibly funny) climax of the tale.
This is a great buy for younger children in terms of story alone, but for people looking to open a dialogue on adoption this might be a really simple way to introduce the concept. It can also be used to start explaining an 'unconventional' family model - duck is a boy after all and still has a baby. That the egg is not his doesn't alter his love for it.
This is possibly my favourite book so far.