Got an email the other day from a quilter whose work I have known about for years, largely because we've been participants on the Quiltart list for a long time, reading each other's messages and blogs. She wrote: "For the last 15 years or so I have concentrated on ways to create portraits with fabric. I am really wanting to grow and develop further as an artist. I am finding that I don't really have a basis for understanding more abstract art -- for instance, why is one 'good' and another is not. I was wondering if you could give me some recommendations of books or websites that might help me develop a more sensitive understanding."
Wow! That's quite a question, and I had to think for a while. I wandered around on Google for a bit looking for some background material, and the best I could find without spending all day was this discussion of abstract art. But I think the real solution to your dilemma is broader than just reading matter, it's how to cultivate a way of looking at and evaluating art, other people's and your own.
I would suggest that instead of being intimidated by how different abstract art is from the representational work you already do, you think about the similarities. Other than not trying to faithfully depict things from real life, abstract art follows almost all the same rules of composition: balance, rhythm, proportion, emphasis, unity. Everything you know about color applies equally to representational and abstract work.
Maybe a good place to start is to look through a bunch of images that you know to be "good" -- for instance, work by the famous abstract expressionists, or the earlier 20th century painters. Run through the list of qualities (balance, rhythm, etc.) and try to see how they appear in these pictures.
For instance, here the yellow and red areas balance the heavy black at the top. There's clear unity in the lines and the grid, and emphasis on the very large white square.
Here there's a rhythm in the lighter pinks surrounding the darker focal center, with white in the four corners. There's unity in the character of the brushstrokes and the quality of the paint (crude, messy, seemingly applied in haste). And so on.
There's rhythm in the size and weight of the five large black shapes, in the strong black verticals and the colored horizontals. The small white and black circles at the right echo the larger ovals. The three columns of the composition have a pleasant proportion, not identical in width but in good balance. The green stripe just below dead center complements the warm earth tones and neutrals everywhere else in the painting.
The more you do this exercise when you visit museums or look at art images online, you'll get better at seeing the principles of good design. And you will be able to see them at play in your own quilts as you compose them on the design wall.
Speaking of design walls, I think that it's important as you become more abstract in your quilts to work with at least some improvisational flexibility. Where making representational pictures in fabric, especially portraits, requires a lot of advance planning and design on paper, and not a lot of changing of plans in midstream, I think you have to give yourself more freedom in doing abstractions on the design wall. Allow yourself plenty of stepping back, looking, contemplating, and letting the fabric talk to you. Colors that you thought would play well together when you made your preliminary sketches or pulled piles of fabric to work with may not be right when you see them in the flesh at actual size.
I don't know whether this answer will be enough to satisfy my friend. Maybe if the rest of you have favorite books or websites or ideas you could suggest them as well. But in the end, I think the solution is to spend as much time as possible looking at and thinking about art, no matter whether you're doing it with the help of books or conversations with others or the internet or some other method. You can develop your biceps by doing curls or chin-ups or working on a nautilus machine -- it doesn't really matter which one you choose as long as you do it regularly. Same with thinking about art.