You have a 6 year old, and a 7 month old baby, and you got through without a sex talk? How in the heck did you pull that off! My oldest was 4 when I got pregnant with my oldest daughter and I couldn't even excape it then!
I didn't use books, I used our normal conversational words, then I used a magazine my OB had given me to show him fetal development. I had some books with human reproductive organ sketches and full body sketches and we talked about those.
I wouldn't make a big deal out of it really. They are still young and do not need to know every little piece of it. Books aren't written per child, so you yourself should write the book for your child. Grab up some magaizines and cut out pictures, or print some from different web sites. Use a cheap cling photo album to make it, you can get them at any dollar store.
Then the book can contain the content you and your child are ready for, and you can put it away for future use.