I get these occasionally from publishers that are hoping I will write a review. There is no obligation on my part to write a favorable review, but it is my nature to go by the old adage, "If you can't say something nice, don't say anything at all." Sometimes I don't say anything at all and that's that. The book in question goes off to The Bookman rather than collect dust on my shelf. This isn't one of those books.
I can see value for the beginning to intermediate knitter in "Customize Your Knitting", especially a knitter that does not have a lot of experience in knitting garments to fit or incorporating shaping into lace and texture patterns. The author goes into good detail on those points.
If you are wanting to take an existing pattern and change out details like sleeve and body length, the style of trim, the type of closure, or the neckline shape, this publication does give guidance in that area. There are also four "classic rectangle" patterns fully developed in sizes small to X-large (generally with a finished chest measurement of 34 - 46") with instruction on how to change the shape to suit your individual body. When you add in the included advice for variation in style details you get the 16 styles shown below.
One negative that I feel must be mentioned; there is not nearly enough said about the impact of various stitch patterns on gauge. When a knitter starts to choose different trims, like replacing a ribbed band with one done in seed stitch, the gauge changes and the stitch count should be adjusted accordingly. There is no mention of this aspect of customizing at all and I saw at least one case where, even in the example sweater, a change in stitch pattern caused unsightly rippling in the edging.
Conclusion? A worthwhile book if you want to advance your knitting past following a pattern to the letter but are perhaps not ready to branch out to design your own from scratch.