I dont know about any good books but I can give you a few tips as my eldest was about the same age when I got pregnant again (its a fab age gap by the way!)
I used to call the baby "your new baby sister" when I talked to her about the baby, which I did often, making her feel that it was as much her baby as ours. I also let her listen to the heartbeat and feel the baby moving. She seemed to understand pretty well that there was a baby in there lol.
Also, once the baby was born, I made sure that my eldest was the first one after me to hold her (with the aid of lots of cushions and daddy supporting her arms) and I got her a present from the baby so she didnt feel left out.
There were many times when I would be feeding baby and my eldest wanted to be on me aswell, and other than pushing her away I let her pretend to be a baby (its what she wanted to do lol) and lie on me too, she used to stroke the babies cheek and make those early sucking noises, it was sooo cute and I felt like it was really important for her to be involved in the bonding process with the new baby.
I hope ive helped some, if theres anything else just ask!
