D.D.
Hi J., I've been there and done that!
There is a book called Beyond One: Growing a Family and Getting a Life By Jennifer Bingham Hull "A unique parenting book focuses on the single-child family preparing to expand to two, three, or more children, covering everything from baby care and sibling rivalry to exercise and sex for the parents in the house."
Very good book and sound like what you are looking for.
Now, My first two children are 21 months apart and the 2nd and 3rd are 15 months apart. It's not tough, but it's not too hard either. In fact now that I've had another after a big gap, I'd much prefer having a bunch of babies in a short amount of time!
My boys loved talking to the baby inside my tummy, even when they were just one. No, they really didn't know that was a baby inside, but it didn't matter. They were able to talk and be involved.
Let me know if you want to know anything else... like I said, I've been there!