There's sometimes a slight overlap between web development and web design, but I don't think web developers should try to be full-time designers unless they're willing to put as much effort into it as they did learning to program. It's not something that you can just dabble in on weekends and be good at.
If this is so you can learn to be your own web designer (i.e. do your own web design for all the sites you work on), then I really recommend you get some formal training (even if it's just taking a few college design courses) to learn the fundamentals of visual design and composition. A textbook-style design book like the one Damon posted could help, but you'll have to actually do all the exercises in the book and make sure you understand the material. But most importantly, you need to spend a lot of time looking at (browse online design galleries, flip through magazines and magazine ads, etc.) and creating designs (download some practice design briefs online or enter online design contests).
If it's to help you communicate with the web/graphic designers you work with, then simply getting a book that teaches you the vocabulary (e.g. typography concepts) and focusing on usability, accessibility, and UX design is probably the best use of your time. This way, you can get involved in the creative direction of the project and help design the interfaces, but the designer is the one who creates the "look" or aesthetics of the site.
Virtuosi Media lists a lot of good resources (the Tutsplus network is great in general, and Smashing Magazine is a must-read for any web developer/designer), to which I'd add: