""Who knows why the Beatles happened?"" John Lennon asked in 1980: if anyone did, it would be the Fab Four themselves, who tell their own storyDwith plenty of visual aidsDin this giant compendium. Festooned with more than 1,300 photographs, posters and documents (many in color), the weighty (6.6 lbs.) volume offers the Beatles' ""own permanent written record of events up to 1970,"" some of it previously published, but much of it transcribed from new or unpublished interviews. Paul, George, Ringo and Beatles-related folks (Brian Epstein, George Martin, Derek Taylor) contribute text from interviews conducted for the book and for an accompanying TV program. Words from the late John Lennon have been gathered from print, broadcast and manuscripts (each with an indicated date), then spliced together to create coherent pages and paragraphs. The book opens with the band members' separate accounts of their childhoods, then moves into a year-by-year organization that allows for great detail and many digressions. Here are what the Beatles have said, or say now, about particular sessions and gigs. Here, too, are comments and reminiscences on every topic in their careerD from marijuana to Manila to Murray the K, from Hamburg to ""A Hard Day's Night"" to ""Hey, Jude."" Most of the text appears oral-history style, in short paragraphs with rapid switches between one Beatle and another: the format makes it sound as if all the Beatles (including John) were being interviewed simultaneously. The visuals bring in cartoons, signed letters, scrawled drawings and photos. As a whole the volume is beautiful, big and a bit intimidating, somewhere between the Yellow Submarine and the Death Star. (One-day laydown, Oct. 5) Forecast: Do people still care? You bet they do. With massive publicity, innumerable tie-ins and enduring, worldwide passion for the music, this is poised to be one of the biggest nonfiction books of the year. Yeah, yeah, yeah!