We've been reading a book called "My mom and dad and I are having a baby" written by a midwife. It is a simple book, from the perspective of a child who observes and recounts his/her mother's pregnancy and home birth. I wish I could get the name of the author, but it's next to the bed.
My son is 2 1/2 and LOVES that book. It has sketch drawings and there is a series of pictures of when the mom is opening up more and more then the baby's head is out. He loves that part the most. He understands all about what will happen, where the baby will come out, what noises I may make, etc.
I'll look to see who wrote it and edit this later. It's old, don't know if it's easily available- we got it from my midwife.
OK- it's written by Maryann Malecki
Here's the amazon link: book
Amazon says it costs like $100.00- I don't understand why-- maybe you could find the author and ask her for a copy of it personally.