I was lucky enough to find The Practical Sketcher: a complete and practical method of sketching, for women’s, misses’, junior’s, children’s and infant’s garments online -and it’s a free download! Published in 1915 by Isidor Rosenfeld, I wouldn’t be surprised if it’s the very first book ever published on technical illustration for apparel.
Using a grid format, it shows proportions of style elements based on human dimensions broken down into 8ths by height (the human figure is close enough to 8 lengths as to not quibble too much about it). While the styles are unarguably dated, I can’t help but think this would be very useful for designers who need to convey style details to pattern makers. For example (upper right), here is this illustration of a back shirtwaist. If you don’t like the flat look, the book also explains how to draw from the 3/4 view. An example of that is shown (lower right) with this view of a shirtwaist front.
With the grid, one can tell where the critical garment attributes lie. [This is not to suggest one wouldn’t later hire someone to do professional looking flats for line sheets but this could be a low cost option to get the style through the prototyping phase to help you determine if you’ll actually produce it for future sales.] Another useful thing about using the grid system is that any separates you draw will be proportionate to each other even if the sketches are rendered on separate sheets. For example, waist length tops are always 3 grid sections and bottoms (in this case floor length skirts but could just as well be pants these days) are five grid sections.
No great shakes but I thought this book might be a guide to simple illustrations for garment proportions that you may find useful. There’s also some cool style ideas typical of the era. All told, it’s 108 pages long. Enjoy!