Low tech isn't a really accurate description; Underhill is working with pre / early industrial revolution technology. Which was the highest tech of the time. And still state of the art for handtool work.
Generally, muscle rather than electricity;skilled work generally done to well fitting precision. Which probably is a better fit to what many of us ascribe to.
Since his work focuses on work done during the era of agrarian society, much of what he builds fits to permaculture ideation
Often, his books 'bootstrap', build a workbench, then use the workbench to build a toolbox, then your kitchen table......... His books give a good description of the process and what to look for, and incentive to get out there and build.
Books often available at your local public library and they show up quite often at used book stores (brick or online).