This book provides a great bit of detail about plain TeX. That alone probably makes it worth the read, even if LaTeX and other derivatives are the flavor of the day. Unsurprisingly, this book provides a lot of useful info about how any -TeX is working in the background.
However, this book also helped to make so many other concepts click for me. It provides a really tactile description of parsing/lexing/etc. But then also it provides great detail and description of typesetting with glyphs, glue, kerns etc. I might even consider re-approaching the Design Patterns book now that it makes more sense.