Solution: C is for Closing Bell

Answer: PROGRESSIVE

Written by Mike Sylvia

The graph consists of 78 nodes, each with exactly one outward arrow. There are exactly twenty-six of each color, and the colors are labeled with names that suggest various alphabetical sets of words: red is The Gashlycrumb Tinies, yellow is the NATO alphabet, and blue is the Kinsey Millhone series of novels.

But which node is which, and what do the arrows indicate? As hinted by the flavortext, each node uses the last letter of the relevant word to determine where to go next — basically playing a game of word chain or shiritori, but always with only one possible option due to the alphabetical nature of the word sets. These links are represented by the arrows. (There's a few oddities in the blue nodes - “X” wasn't “for” anything, so it's ambiguous whether its arrow should go to itself or to nothing at all, and Sue Grafton sadly passed away before writing the twenty-sixth book, but she did state that the planned title was “Z is for Zero”.)

Not all nodes can be uniquely determined - for example, WHISKEY and X-RAY’s nodes look identical on an unlabeled graph, since they have no feeders and both feed into YANKEE — but one entire row of the grid can be unambiguously filled in, spelling the answer PROGRESSIVE.