LITTLE ROCK, Ark., Nov. 19 - With its sleek horizontal form hovering at the edge of the Arkansas River, the new William J. Clinton Presidential Center has been called by promoters a "bridge to the 21st century," a trite allusion to one of the former president's favorite themes. Locals snicker that it looks like an enormous double-wide trailer. Actually, its best elements fall somewhere between those two extremes. Designed by James Polshek and Richard Olcott of the New York-based firm Polshek Partnership, the library has moments of genuine architectural power. Its sleek cantilevered form thrusts out aggressively toward the river,...