Four “previously unpublished scenes” accompany the paperback edition of David Foster Wallace’s unfinished novel, “The Pale King,” which arrives in bookstores this week. Three of them are nice for any Wallace fan to read and keep, but are not essential to our understanding of the novel about a group of I.R.S. agents working in Peoria in the nineteen-eighties. The last literary “bonus track,” though, is a keeper. And, at fourteen pages, it’s also the longest new piece of unpublished Wallace fiction to emerge since “The Pale King” itself.