There are two:
And they are both fairly similar. Both develop knitted pieces by combining squares and triangles in a variety of ways. The real difference between the two is the yarn they select for the various projects described in the books.
1. The Modular Knits book by Iris Schreier uses multi-color dyed yarn which is knit fairly loosely in a garter stitch. Most but not all of the colorful pieces are knit with one yarn which is dyed with short bursts of color to acheive the multi colored appearance. The author is part of Artyarns which sends me 'homework' once a month on modular knitting. I already did the first assignment to knit a right triangle attached to an equilateral triangle. I need to finish the second tutorial. This is how I am learning modular knitting... a little bit at a time. The hand dyed yarn used in the Modular Knitting book is from Artyarns. I am going to dye some yarn more to my liking in April and knit a scarf in this technique and see if I like it.
2. Domino Knitting by Vivian Hoxbro gets the multi-color effect by using solid color yarns and changing the yarn during knitting. Domino Knitting also uses a lot garter and a bit of stockinette in the square patterns.
By knitting a few potholders from Domino Knitting or a scarf from Modular Knits one can easily figure out the technique. Its basically knitting small shapes together while the work progresses. From reading parts of the books, Domino Knitting seems to be more time consuming while the Modular Knits projects go rather quickly. There are no projects in either book that leap out and says 'Me! Me! Knit Me!' so back to the library these books go.