I can recommend this book, it goes into a bit more depth regarding plays.

https://amzn.asia/d/djErzDC

There's also the perfect play repository here;

https://muehle.jochen-hoenicke.de/

Once you have some basic plays, follow them through on as many different pathways as you can. Really good players have great memories, and know how to respond to many plays.

I recommend to start with

1. The book Nine Men's Morris Strategy: A guide for beginners to intermediate players, it goes into a bit more depth regarding plays.

2. A perfect play repository by jochen-hoenicke

Once you have some basic plays, follow them through on as many different pathways as you can. Really good players have great memory, and know how to respond to many plays.