Show Boat-lovers--except for the author's fellow musical-theater historians--will probably ignore Kreuger's dry exercise in scholarship and soak themselves instead in the Mississippi flood of photographs, sheet music, sketches, and theater memorabilia that irrigates this handsome celebration of the Kern-Hammerstein show's 50th anniversary. Not that Kreuger's contribution is devoid of interest. In dutifully following Show Boat chapter by chapter, scene by scene, change by change, from bestseller to Broadway to musical films to stage revivals, he turns up a few little-knowns and never-thought-abouts: the lost 1929 film version; the embarrassing material cut in out-of-town try-outs; Oscar Hammerstein II's incognito role as stage director; and how the opening syllables of ""Ol' Man River"" moved with the times from ""niggers"" to ""darkies"" to ""colored folks"" to ""here we all"" to ""nobody."" But, except for his sarcastic scorn for the 1951 Hollywood version (Ava Gardner as ""exciting as an apple core,"" Kathryn Grayson's ""gurgly, thin, reedy soprano"") and except for his plea that Show Boat be performed as originally conceived, Kreuger injects his impressive accumulation of names, dates, and facts with no theatrical spirit and no insights into the creative process. Still, to gaze at Lucinda Ballard's costume sketches, to hum along with ""Bill"" (in both the original P. G. Wodehouse version and the Hammerstein rewrite), to look again on Paul Robeson and Helen Morgan and Irene Dunne--'tis magnolia paradise enow.