Everyone thinking about designing their own knits should have access to a stitch library. Having a few books to leaf through (and some good websites too) is an invaluable source of inspiration.
(Lace ribs 1 from craftcookie.com)
Usually, when I first have an idea for new socks, I tend to know how the overall construction should look, but I won't always know exactly what kind of stitch pattern I want. For instance, I'm currently working on socks that I wanted to have a ribbed cuff, but not the usual k2 p2 cuff. So I went through my stitch library and online sources until I found something that appealed to me and would be easy for knitters to memorise.
(Bubbles from Vogue Knitting)
When you look to buy a good stitch library, make sure it has a wide variety of stitches. There should be a number of sections including lace, cables, twisted stitches, colourwork etc. Ideally, there would also be a section on edgings and perhaps finishing too.
(Rib and welt from Patons Yarns)
I like books that have both written and charted instructions, though if I had to choose, I'd always pick the charts. That's an entirely personal preference, of course, but charts are great to help you visualise the pattern.
(Daisy stitch from knittingfool.com)
Do you have any recommendations? Is there a book or website you have found particularly useful and interesting to look through? Feel free to share your tips in the comments.