I'm not sure it does cover plain (non-tartan) tweeds. I'm pretty sure it's using "tweed" to mean "that wool that scots make kilts of". It's not really useful information on making kilts, though. The practical cutter's books are interesting, but they're not very useful for most people. The target audience was apprentice cutters -- who would already have done years sewing up garments as part of their tailoring apprenticeship, and so know how things are made -- and are now learning how to cut the cloth to make them. So they're not very complete instructions, there being lots of detail that the reader is assumed to know.