I found that many books intended to prep children for a new sibling assume that jealousy will be a part of the adjustment process. I do not find this helpful. In my experience, with my kids and also in other families I've observed, if babies are presented as something to love, serve and value, kids will follow suit.
These are our favorite 5 "big brother/big sister" books:
1. Will There Be a Lap for Me?, by Dorothy Corey-- I love the realism of this book. A boy notices his mom's lap getting smaller and misses sitting in it. Of course his mom is pregnant and there is a sweet picture of him feeling his sibling kick and looking forward to the birth (so very positive in that way). But even when the baby is born (a brother), mom's lap isn't so available-- the baby is so needy! I love the line about the baby being hungry "all the time," showing Mom nursing. Anyone who has nursed a newborn knows this is reality. I also loved that the diapering is cloth diapers! But as the baby gets bigger his big brother is able once again to get time with Mommy just like they did before the baby was born: they swing outside watching the birds, with him on his mother's lap.
2. On Mother's Lap, by Ann Herbert Scott-- this book is set in a modern Eskimo village, which is something different and subtly shows the universality of family, maternal love, and siblings. It is not directly about welcoming a new baby, but addresses the question siblings could bring about "is there room for both of us?" This book gives a resounding YES; there's ALWAYS room for you on Mother's lap.
3. Ben's Baby, by Michael Foreman-- in this story, the big brother asks for a baby for his birthday, and the whole year is spent preparing to welcome the answer to his prayers. By his next birthday, his wish has come true!! So positive and sweet. Beautiful illustrations.
4. Our Water Baby, by Amy Maclean-- this one was particularly helpful for us this time around, because we planned a home water birth just like the family in the book, however even families planning to have a birth center or hospital birth, or a non-water birth, can find it helpful. (Just say something like "some mommies labor and give birth at home, other mommies go somewhere else for the birth, or the birth and the labor.") It is very positive, portrays birth realistically without being scary or graphic ("Mommy is doing hard work to get the baby out; she will feel better when the baby is born."), shows breast-feeding and best of all, shows a family purely welcoming a new baby. The big brother imagines what he will teach the baby, not what the baby will do with/for him. I also like that the baby is expected in a general season ("when the roses bloom") instead of a date.
5. Peter's Chair, by Ezra Jack Keats-- Keats' illustrations are splendid as always, and he captures the emotions and family dynamics so well. Peter's old crib, cradle and high chair have all been claimed by his new sister, so he selfishly grabs his little chair and runs away... however when he realizes he can't even fit in the chair anymore, he comes back home and paints the chair pink himself. I like that this does show some negative emotion-- selfishness over hand-me-downs-- but the big brother sees the folly in that all on his own.
I'm a Big Brother, by Joanna Cole-- I find the newer version with Rosalynn Knightly's illustrations to be preferable to the older version, but neither are what I would ideally choose. I LOVE the positive tone in this book. Drawbacks are that the mom still hardly holds the baby, bottle feeding is the only form of feeding shown, and the baby is mostly in a carseat-type carrier.
What Brothers Do Best, by Laura Numeroff-- this shows many things a big brother can do for a smaller sibling -- just make sure that it's clear that a new brother or sister won't be able to enjoy any of these things right away!