If you read the Happy Hollisters series as a kid – or if you ARE a kid – several of them are free right now at Kindle US and UK. These are from early in the series: books 3 – 7, and 9. (No idea why they skipped #8, which is still regular price…)
The Happy Hollisters at Sea Gull Beach, #3: Kindle US | Kindle UK
The Happy Hollisters and the Indian Treasure, #4: Kindle US | Kindle UK
The Happy Hollisters at Mystery Mountain, #5: Kindle US | Kindle UK
The Happy Hollisters at Snowflake Camp, #6: Kindle US | Kindle UK
The Happy Hollisters and the Trading Post Mystery, #7: Kindle US | Kindle UK
The Happy Hollisters and the Secret Fort, #9: Kindle US | Kindle UK