Who's the arsonist on the loose in Civil War-era San Francisco? That's the mystery this time for the heroes of The Mark Twain Murders--journalist Mark Twain (the very same) and the 15-year-old urchin/narrator who calls himself "the Duke of Baywater." The Duke, hungry and prowling around warehouses for valuable trash, witnesses an explosion at a chemical factory; the only clue is a nearby van bearing the name of a photographer. So when the fire attracts newsman Mark Twain to the scene, the co-sleuths again team up, with assistance from mustachioed fireman Tom Sawyer. They're just in time to see another fire break out during the parade welcoming a star-actress ("the Pritchard") to the local theater. And, just by coincidence, the theater's tart/sweet old costume-lady, Letty Cleary, has a connection to the mystery-photographer--who seems to be planning bigger, more dangerous fires, aimed at San Francisco's pro-Union establishment. Could it possibly be, then, that the arch-villain is once again Major St. John, the Confederate spy who plotted to rob the San Francisco Mint in The Mark Twain Murders? Yes, indeed, the presumed-dead Major--"so cold and calculating that he was beyond maean"--is still alive, posing as a photographer, and up to no good. He captures the heroes briefly, coolly kills dear Letty. (Her death is a little disturbing in such a light, comic-booky caper.) Escapes, gun-waving, and another capture ensue--as the heroes chase the Major through a society costume-ball. But, though locked up in a burning room, the good guys escape again. . . in time to defuse all the arsonist's infernal contraptions. As before, Yep gives the real Mark Twain little color or substance; the Tom Sawyer character is lackluster too, with no kinship to the fictional lad. (According to the foreword, there was a real S.F. fireman who claimed to have inspired Twain's creation.) But this less elaborate sequel is another bright, quick, folksy adventure--at its best in the period touches, especially the firefighting details.