My kids are 4 years apart.
My eldest, my daughter, was 3 when I was pregnant with her brother.
For me I didn't use books.
I simply talked with her and spent my entire pregnancy, on prepping HER.
I explained to her, in simple terms, what a baby is, what it does, that it cries and wakes at all hours, that Mommy breastfeeds him, that Mommy will take care of him and wake up so she doesn't have to "worry"... and that HER routines will not change.
Also, every month, I took photos of her WITH my growing belly. She loved that.
She also talked to my belly (her brother) and sang to it.
I also took her to ALL of my prenatal appointments (which my OB/GYN encouraged) and he even taught her how to use the Doppler heart monitor on my tummy. She loved that.
I spent all of my pregnancy, prepping her. It was not just about "me."
It was about her.
And when her baby brother was born... she was all adjusted. She adjusted like a Champ. And she was ALREADY bonded with him... because she had bonded with him even while he was in my tummy.
I ALSO explained to her, that once she has a sibling, that her things are HERS. I will not expect or 'force' her to 'share' EVERYTHING. Its okay. But I always told her to tell me anything about how she feels. I am there for her. I also told her, I will NOT make her an 'example' for her baby brother, nor expect her to be all grown-up... just because she is the eldest. That I KNOW she is a child herself.
We also made, special hand-shakes and head nods, between us. So that... when I am busy with her baby brother or breastfeeding, she CAN do that with me and feel special. Even if we are across the room from each other.
ALL of these things, helped my daughter adjust.
It reassured her.
And she KNEW... that it was okay to just be herself. Even if going through her own, age-related phases.