Not a book, but a super touching kid song by Frances England about an only getting a new sibling is Family Tree. I definitely recommend it.
Let's see, we like:
We're Having a Homebirth by Kelly Mochal
Hello Baby by Jenni Overend
What Baby Needs by Dr. Sears
Baby on the Way by Martha Sears
We Have a Baby by Cathryn Falwell
Being Born by Sheila Kitzinger (I used this with my 2.5 year old... we didn't read all of it, but the pictures are great!)
Christmas Is Coming by Anne Bowen
Prairie Christmas by Elizabeth van Steenwyk
Happy Birth Day by Robie H. Harris
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