The Book of Audacity: Record, Edit, Mix, and Master with the Free Audio Editor by Carla Schroder

My rating: 3 of 5 stars

Some of the software details are out of date as Audacity has continued to evolve since the book was published. However, the information and theory behind why you would use certain plugins or hardware remains good background information. I think if you are able to learn that information you can apply it to the modern interface and plugins.