Five books about sleuths

Gwenda Bond and Christopher Rowe are married, and the Supernormal Sleuthing Service series is their first writing collaboration. Among their five favorite bookish sleuths, as shared at Tor.com:
Harriet the Spy by Louise Fitzhugh

I [Christopher] loved this book so much as a kid, I got in trouble for carrying around my own secret diary modeled on Harriet’s. To be fair, my observations of my