Books can be a great way to ease into a difficult topic.
Simply sitting down and reading a book with a child provides the opportunity and the vocabulary for the child to discuss issues.
They may not say anything the first five times you read the book, but sooner or later, there will be discussion.
Be patient and let your child initiate it.
Children adopted at an older age have all experienced grief and loss and will need help to process it.
What kind of loss? The loss of a first family, the loss of caregivers, the loss of a dream of a "perfect" or "normal" family, the loss of a school (or two or three or more), the loss of a sense of continuity, a sense of belonging, a sense of being regular...
the list goes on and on and is different for each child.
What is the same for all children is that they need to discuss this and feel your support and understanding.
Sometimes talking about this (again and again and again--it's a process) will be enough.
Other times, the help of a therapist will be required.
Here are some books that can help:

1. Lifetimes: The beautiful way to explain death to children by Bryan Mellonie and Robert Ingpen.
   Very simple story, not many words.
   Explains the different life expectancies of different animals.
   Gentle and general.
   
2. Sad Isn't Bad: A Good-Grief Guidebook for Kids Dealing with Loss by Michaelene Mundy.
   My daughter brings this book everywhere with her.
   A wonderful self-care guide for kids who have experienced loss, with advice such as: It's okay to cry, it's okay to ask questions, it's not your fault, and ending on a fairly upbeat note--that healing will happen, that it's good to remember.
   Very appropriate book for kids who have experienced loss but not death.
   
3. Tear Soup by Pat Shwiebert and Chuck DeKlyen.
   Described as "a recipe for healing after loss", this wonderful book talks about all kinds of grief, including divorce, and makes it into a soup.
   Happy memories are what makes the soup taste better.
   Really well done.
   
4. The Next Place by Warren Hanson.
   While about death, this is one of the most gentle, uplifting book I've ever seen.
   With gorgeous illustrations, the first three-quarters of it discuss loss and would be very appropriate for any type of loss.
   The last few pages specifically discuss death in a calm, not scary way.
   Lovely.
   

Two classics that most adoptive families can benefit from are both by author Todd Parr:

1. The Family Book.
   Shows families that are different colors, formed by adoption, stepfamilies, big families, two moms or two dads, etc.
   Points out the commonalities that all families share.
   Inclusive and accepting.
   Delight-filled.
   
2. It's Okay to Be Different.
   Wonderful message that all our many differences: different nose, needing help, being a different color, needing a wheelchair, talking about feelings, having different moms or dads, being adopted...
   the list goes on...
   they are all okay.
   With Todd Parr's simple illustrations that somehow make the message even more credible.
   Kids love his books.