I came across two loop-related articles in two separate publications. One was a cover story in Signal to Noise about Pauline Oliveros. It discusses her early work with the SF Tape Music Center as well as recent works with delay and reverb. The other article, in Magnet, is on Terry Riley. It too discusses the SF Tape Music Center as well as other works that are quite loop-oriented. Both these magazines are hard to find. Here are links to their websites if you're interested in checking them out.