Pattern, like texture, is used to create visual interest. Unlike texture however, pattern does not invoke a tactile sense. Although textures can contain pattern-like elements, not all patterns are textures. Here patterns and variation are used to create interesting shapes with unity.
References for various fundamental things - composition, balance of details etc. I don't have sources for most of these as its something that was lying on my hdd for a while, but you should be able to look it up through google images.