Hi Nicky
there is a series of books by Jeanette Rowe. One of them is called 'my new baby'.
I don't have that one because my DS1 was only 13 months old when his little baby brother was born so he wouldn't have understood, so I can't tell you what's in it, however, I really recommend her books. I have 4 of her books - My mum, my dad, my big sister and my big brother and we read them to our sons every day. My DS1 is only 2 years and 5 months old, and he already memorises which book has which sentence, and honestly, can even read them to me. I bought them from the ABC book shop, and I have seen a few on ebay but you're better off with the bookshop coz they can order them in.
hope this helps
Haylee.
3 gorgeous children to love forever