What to watch on PBS this fall season -- We look at the network's latest war documentaries, historical pieces, and detective series

Sit up straight, take that gum out of your mouth, and open your notebooks: PBS is beginning its 1990-91 season with a five-night crash course in American history. In The Civil War, airing Sept. 23-27, documentary filmmaker Ken Burns has stitched archival photographs, interviews, and dramatic readings into a 12-hour collage that offers TV’s most comprehensive account of the War Between the States. Dozens of familiar voices, among them Sam Waterston as Abraham Lincoln, Morgan Freeman as Frederick Douglass, and Garrison Keillor as Walt Whitman, will help viewers through a maze of advances and retreats.

The military mood continues with Korea: The Unknown War (Nov. 12-14), a six-hour history coproduced by British, Australian, and American television. But public broadcasting’s other major undertakings will deal with even more basic matters: earth and sky. The 10-hour Race to Save the Planet (airing on consecutive Thursdays starting Oct. 4 and in a block Oct. 7-11) will look at environmental solutions being implemented around the world. Meryl Streep introduces each of the episodes, which are narrated by Roy Scheider. And The Astronomers, a six-hour look at history’s stargazers, is set for January.

In its 20th-anniversary season, Masterpiece Theatre won’t have any of the sumptuous, epic adaptations of 19th-century classics that made its name. Instead, viewers will see Jeeves and Wooster (Nov. 11-Dec. 9), a comedy series based on P.G. Wodehouse stories; The Ginger Tree (Oct. 14-Nov. 4), a historical drama set in Manchuria and Japan; and one-shot movies based on Elizabeth Bowen’s The Heat of the Day (Sept. 30) and Evelyn Waugh’s Scoop (Dec. 30). Series executive producer Rebecca Eaton promises Summer’s Lease, a multipart comedy-mystery with John Gielgud, and Portrait of a Marriage, a biography of writer Vita Sackville-West, for early 1991.

Mystery!, PBS’ most popular drama series, will offer new episodes of many recent detective shows — Sherlock Holmes, Poirot, Campion, and Inspector Morse — and also several crime miniseriis. Series host Diana Rigg stars in Mother Love (Oct. 25-Nov. 8), a story of parental affection that warps into malevolent behavior; and Die Kinder (The Children) casts Frederic Forrest and Miranda Richardson in a six-hour drama about kidnapping and international terrorism. Finally, American Playhouse begins its 10th season in March with a lineup that includes taped versions of two recent acclaimed Broadway productions: Stephen Sondheim’s 1987 musical Into the Woods and this year’s Tony-winning drama, The Grapes of Wrath.