1. Many color reproductions, with close-ups (as if you were looking at it under a loop). So useful for print ID. The visuals he includes are an integral to understanding his text; they're not "obligatory image after 50 pages of dry academic writing" visuals.
2. Graphics showing all the layers in a screen print:
3. Asides which are isolated in gray boxes are some of the most interesting bits in the book. This design strategy allows for clear writing, unencumbered by footnotes, without forcing the author to cut interesting extraneous information. For example, this spread is about silk production:
4. Full size reproductions of many pages from a 1927 Selectasine screen printing manual:
5. Detailed descriptions of how stencils, inks, and presses were made. For example, photosensitive emulsion was first detailed in an article by William Hugh Gordon in 1916 in Signs of the Times. In 1926, he released his formula for the emulsion: gelatin, water, egg white, glycerine, and bichromate. Although Lengwiler includes a lot of technical information, this book would be also be accessible to non-printers.
I don't think English is Lengwiler's first language, which makes his excellent writing even more impressive. My favorite parts of the book were the beginning, which details commercial stenciling before screen printing, and the end, which talks about screen printing of textiles, glass, and ceramics.
Lengwiler seems to be a bit dismissive of screen printing as fine art; he could have written about modern artists working in that medium.