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If you really want to learn knitwear sweater design, these are essential, must-have books:
The Knitter’s Guide to Sweater Design by Michelson and Davis – It’s out of print, so you’ll have to hunt to find a reasonably priced copy. Keep an eye out, because it is worth every penny.
Knitting from the Top by Barbara Walker – What you really need to know for top down garment design.
Though both of these books refer to knitting, much of the information is applicable to crochet sweater design as well.