Once, Borders was, with rival Barnes & Noble, the long tail of reading, with supermarket-size stores offering thousands of obscure titles alongside bestsellers. Now, Borders confronts the limitless, more efficient supply chain of Amazon’s online emporium. Borders, which helped a generation of readers learn the pleasure of diving into a book for hours at a stretch, now competes for the attention of readers who dip into a few pages on an iPad, open Facebook, read some more, then tweet random thoughts.