The case of the missing Encyclopedia Brown movie is explained by Mental Floss. Cutting to the chase a bit:

Finally, Deutsch reached a deal with HBO in 1988. The network that put Fraggle Rock on the map was interested in expanding their children’s entertainment brand and ordered a live-action Encyclopedia Brown special that led into a recurring series. Producers filmed the pilot in Provo, Utah, and the episodes were well-received.

Deutsch then did something unexpected. After just six episodes, he insisted on breaking away from the network, which puzzled them. “The idea of a producer taking his show off the air that was successful, that was so good, and so far ahead of its time that it made my career is [mind-boggling],” show co-producer Ned Kandel told The New York Times in 2005.

There’s a lot more drama in the article.

I loved these books when I was a kid. I’m pretty sure I bought them with my own money at the school book fair. I don’t remember which ones.