I told my daughter pretty soon after I found out. I was 8 weeks along at that point and told her a baby was growing in mommies tummy and she was going to be a big sister. Until I started truly showing did it start to sink in. She was 28 mos when I found out.
I never read any books really. Seriously it all depends on your childs temperment. My daughter wanted a little brother or sister so badly she was so happy and excited. There are no magic tricks to making kids close. Takes time and depends on their personalities.
My son worshipped and still does, his big sister. She was very good with him until he got verbal and mobile then it was a fight every day over little things, now they are 4 and 7 and the fighting has lessened a ton. She never had one ounce of resentment or jealousy and it is nice as a newborn sleeps a lot and I had tons of time still with her during the day. When my son was very little I put her in one day a week Moms Day out so she had something of her very own without being associated with the baby. That helped a lot. It is a change and kids handle change differently too. She regressed with potty training a bit but that was all I noticed in her.
I took her to every OBGYN appt, let her hear the heartbeat, see the monitor and all so she really got what was happening.
The night before we brought her brother home we got her a boy baby doll all of her own, with accessories so she could take care of her baby while I took care of mine. She was very happy and it was a breeze of a transition. I also sat with her before I went to have him at the hospital and showed her baby pictures and videos of herself. Explained how much care babies need and why they cry.
Funny thing is when her brother cried she would yell at me to JUMP NOW and take care of him and was very doting.
Not until he was 2 and into her stuff and pulling her hair did it occur to her maybe she wasn't too thrilled with a little brother, hee hee.....now they play well but fight too!
Congrats!