Many of the books written to explain adoption to adopted kids can also be used to prepare kids already in the family for the arrival of an adopted sibling. I particularly like the ones that focus on how a family is a family regardless on how its members joined. Also, your library has many books on preparing any kid for becoming a big brother or sister. The books I've listed here specifically address adding a sibling through adoption.

• Seeds of Love: For Brothers and Sisters of International Adoption by Mary Petertyl (2-8)
• Things Little Kids Need to Know by Susan Uhlig (2-6)
• Is That Your Sister by Catherine and Sherry Bunin (5-10)
• My Special Someone by Brittany and Sherry Kyle (2-7)
• Just Add One Chinese Sister: An Adoption Story by Patricia McMahon and Conor Clarke McCarthy (4-8)
• My Mei Meiby Ed Young (2-8) Fantastic art work. Both girls are adopted from China
• Jin Woo by Eve Bunting (4-10)
• Emma's Yucky Brother by Jean Little (5-10) Great for families adopting a toddler or older child to help prepare the older siblings. The family is adopting from the foster care system in the US, but this book could be adapted for a family adopting an older child internationally.
• Waiting for May by Janet Stoeke (5-9). This story follows a family’s long adoption journey to China and focuses on the older brother to be’s feelings and emotion both during the wait and when they finally meet May. Briefly, the mother explains how a birth mother might feel. The brother is disappointed when May cries continually and clings to her caretaker. Finally May becomes intrigued by her big brother’s buttons and allows him to hug her. Great book.
• A Sister for Matthew: A Story About Adoption by Pamela Kennedy (4-8) This is a nice book to help prepare your younger kids for the adoption of a sibling. It addresses the natural concerns they may feel, and is a good conversation starter. Although it is listed for up to age 8, I think most 8 year olds would find it a bit babyish unless they are listening in when you read it to a younger child.
• A New Barker in The House by Tomie dePaola (4-7) This is one of the continuing books in the Barker Twin series. In this book the parents are adopting a Spanish-speaking toddler. At first the new child is overwhelmed by his rambunctious siblings, but soon they all settle in to become a family.
• I'm a Big Sister or I'm a Big Brother (2-6) by Joanna Cole and Maxie Chambliss. These are not adoption specific, but since they don’t cover the pregnancy and hospital part of becoming an older sibling, they can be used by adoptive families. There is also a section for parents.