Congratulations! Such an exciting event for you all.
Books are age dependant. We struggled to find anything appropriate for our 4 year old BC, most too babyish, and others were American (different adoption scenario) or dealt with inter country adoptions.
I think there's a big gap in the market.
What we did do was compose and send an email that we sent to all friends and family, explaining what was happening, what we would be doing with regards to introducing our LO (funnelling at first so slow wider introductions), what information we would and wouldn't be sharing with them (background to LO's adoption) and what message we would be giving our BC and her school friends.
We then sent a separate (lighter) email to all parents of BC's school class explaining the message we had given BC about LO.
I'm so glad we did that as we had underestimated how hard some parents might find it to explain.
Our message was this: Some mummies and Daddies find it too hard to look after a baby, so they are adopted by a family who can.
The important thing for us was that we got that message out there ahead of LO coming home.
One of our very good friends (in fact one of our referees) who had been with us every step of the way got herself tied up in knots trying to explain it to her children before we had the chance to send the email. They couldn't understand "why her mummy wouldn't want to keep her"
Actually this was not the case and not the message we wanted LO to ever hear when she is old enough to understand.
Sorry for the waffle and good luck