Sashiko: Blue and White Quilt Art of Japan, Kazuko Mende and Reiko Morishige (1991). Not a how-to treatise, this book instead places sashiko into a historical context and details the significance and usage of various patterns. For example, Uroko [fish scale] patterns are said to be used on Noh theater costumes "to represent the transformation of beautiful women into dreadful, jealous characters, consumed by revenge"; one might thus assume that it wouldn't ordinarily be appropriate for a gift to a traditional Japanese friend!
This book's strength to me personally, however, lies in the transitions: striking examples that are not content to show one beautiful but static pattern at a time. Instead, pattern flows into pattern, or a pattern is repeated using a different scale or alignment, or an otherwise evenly repeated pattern irregularly disappears at its outer edge into the vast indigo blue. This is an inspiring book; I wish it weren't OOP (and $$$).
Sashiko: Easy and Elegant Japanese Designs for Decorative Machine Embroidery, Mary S. Parker (1999). Here is the how-to manual. After an initial historical section, the book dives into "secrets of successful sashiko stitching by machine" (including advice for choosing threads, transferring patterns, and more), followed by sample projects and a stitch pattern dictionary; a bibliography and an index complete the work. The projects range from "swatches" of coasters and hot pads to intricate sampler-style screen panels, and also show how sashiko can be used to good effect on garments. The colors are not limited to traditional indigo and white, either, but instead vary as much as any other embroidery might.
The most useful part of this book is, to me, how the pattern dictionary employs colored lines to show stitching order, which has been tailored to machine stitching by minimizing stops and starts. Parker also suggests bobbin stitching to allow a heavier thread on the right side of the fabric, and adjusting the tension so the threads lie slightly to the wrong side of the fabric; these increase the sashiko feel. I don't know yet whether I want my own copy; that will depend on Briscoe's The Ultimate Sashiko Sourcebook, which I haven't yet seen in person to know whether it is indeed as "ultimate" as it claims.
Japanese Country Quilting: Sashiko Patterns and Projects for Beginners, Karen Kim Matsunaga (1990). Some might not find this to be in fact a beginner's book, as it gives the bare bones of tools and techniques, history and design considerations; although the information is sufficient to get going, less confident beginners might prefer a more thorough treatment. (Not to be missed is the technique of avoiding knots via "overlapping" stitches, essentially making two to three stitches and then going back over them once, piercing the original stitches' thread with the needle along the way so as to interlock the fibers.) There are five projects, all of which are given historical context and are interesting in and of themselves, more so perhaps than the names might indicate: a "Knickknack Bag," "Triangle Bag," "Floor Cushion," "Doorway Curtain," and "Wrapping Cloth."
Its considerable strength, and the reason I bought this book some years ago, is that it's wonderful for designers. Not only does the pattern dictionary clearly show each pattern along with a historical note and, often, variations, but it also includes drafting instructions for use with graph paper. There are also notes on stitch order, meant for hand stitching, and practical notes such as that "Overlapping Diamonds" is "moderately complicated to draw yet easy to stitch" or that "Hemp Leaf" "requires many knots and so is best for projects that will be lined." If you want to customize your designs, this book is for you... and it's even in print.