I've had a couple e-mails from different folks asking me for suggestions, these last couple weeks. As a rule when I get more than two requests for the same information, I publish it here (unless it's about penis size enhancement and the information requested is my credit card number). So here are the books I use most when working out my own designs. Please know, these aren't the ONLY books out there, they aren't even the only GOOD books out there. They're just the ones I use the most. I'll try to explain why, with each one. If anyone has different favorites, give a shout out in the comments. I'm always on the lookout for a good book.
First and foremost, the book I'm constantly going on about around here, and is currently laying open on my desk:
"Knitting in the Old Way" by Priscilla Gibson-Roberts and Deborah Robson. If I could only have ONE reference book on sweaters/cardigans/pullovers, it would be this one. Technically, it is only about traditional knitwear, and puts it all in the context of the EPS, or Elizabeth's Percentage System. That's the one worked out by Elizabeth Zimmerman, which uses the chest measurement as 100% and all other measurements are percentages of it. There are quite a few different construction methods discussed, and with a bit of creativity, you can apply this to all the rest of designs, traditional or not. Because, while the traditional sweaters are all knit bottom-up or top-down, you can work the percentages out in centimeters instead of stitch count, and then use the dimensions to knit in any direction you like. (Yes, technically you can work out the percentages in inches, but doing that is a royal pain in the ass.) Years ago, when I was knitting a sweater by Elsbeth Lavold, I found an error in the sleeve caps. It was this book I used to correct it; I figured out how high they SHOULD be, in cm, and then knit it to fit. It's a very useful book. It also has hordes of information on steeking, different fiber types and what they're good for, different knitting methods, a survey of traditional sweaters from all over, and basic patterns for at least two dozen. You could knit out of this book for a decade, easy, and not knit the same thing twice.
"Designing Knitwear" by Deborah Newton. This was written back in the eighties, I believe. It's been around for a while, for sure. And it's still in print, because it's a really good book. It talks through the creative process, how she starts with a sketch, how she decides what yarn to use, what color, what everything. And then how she works out from there. From Ms. Newton I gained the habit of drawing out my designs on graph paper, with one square equaling one inch (usually) and from there if you get the math it's all down hill. There are several patterns in here to give you a final product to look at, and a great deal of discussion on how she got there.
"Knitting from the Top" by Barbara Walker. This one covers ALL garments, not just pullovers. ("Knitting in the Old Way" is upper body garments only; "Designing Knitwear" touches on dresses and other things, but not much.) Pants, hats, skirts, dresses, you name it, it's all in here. Combine it with the EPS and you can knit the world. There's a section in the back of general comments from Walker that reads like the notes from a master class on knitting. It's worth the cost of the book for those five or ten pages, alone, even if you never design anything. (Stuff on gauge swatch, fiber choice, working a project, etc.)
"Meg Swansen's Knitting" by, well, Meg Swansen. This isn't, technically, a design book. It's a book of patterns. But there is a technique section in the front that is worth the cost of the book. And each pattern is written in her mother's style, with all sorts of sidebar commentary about how she chose to make the garment the way she did, and, well, a glimpse into Meg Swansen's brain? Always educational and helpful and fascinating.
There are probably another dozen or two books I use when designing, some of them not even having to do with knitting, depending on where I found my inspiration. Fashion history is always educational, and books on sewing can teach you quite a lot about garment construction - especially books that discuss sewing jersey (tee shirt material). But with the books I've listed above, I think you'd be able to make just about anything you want to.