I know exactly what you mean all the books etc about helping siblings deal with the arrival of a new baby are focused on older children.
My boys are 17 and a half months apart, during the pregnancy I would talk to him about the baby and say where's the baby and he would pat my tummy (and sometimes his own!) when ds2 arrived we had a present for ds1 from the baby for him at home when we came home from the hospital, I think alot of it is trying not to focus all your attention on the new baby so they don't feel left out.
Ds1 also came to visit us in the hospital and when we came home from the hospital ds1 came in and all four of us went home together, I thought it was important that he knew we were all going home together as a family.
I don't think he really comprehended the concept of a baby coming but I think if you involve them with the baby, getting nappies or clothes out, helping with bathing etc and still give them attention they need they are more than happy, ds2 loves his baby brother and has just fitted in perfectly almost as if he was always there.