Hi A.G.,
you've received a lot of good ideas on how to get books on the cheap!
Here's a list of a few great book(and some series)
1. Concord Cunningham, The Scripture Sleuth by Mat Halverson- its a lot like the Encyclopedia Brown books but uses his knowledge of the Bible. Can buy the books for cheap on Amazon, have her read them then resell on Amazon etc.
2. Animal Friends series by Janette Oke- 15 books in the series about life lessons & friendship, sad the books are out of print but check used book stores/eBay/Amazon.
3. The Ruby Slippers series by Stacy Towle Morgan- published back in the 90s but can possibly be found at library or 3rd party sellers. About 2 sisters that are home schooled and love to join their dad on his works trips to places all over the world and they learn about God, other cultures, & life lessons.
4. The Adventures of Lily Lapp by Mary Ann Kinsinger and Suzanne Woods Fisher-adventures of a small Amish girl.
5. Cul-de-sac Kids by Beverly Lewis- lighthearted chapter books about neighborhood friends learning about faith, teamwork and they go on adventures.
6. Adventures in Odyssey's The Imagination Station series- very educational time travel stories, the children travel to different periods in history.
7. The Three Cousins Detective Club series by Elspeth Campbell Murphy- mystery solving adventures of 10 or old cousins. They learn about fave Bible passages while solving mysteries.
8. In Grandma's Attic series by Arleta Richardson- old series recently reprinted. About a young girl as she listens to her grandma's stories of the old days. Stories of laughter, discovery, mischief and learning about God.
Good luck on your book adventure!
P