IF ONLY

Don't look here for the scoop on why the star formerly known as Ginger left the Spice Girls so abruptly last year (''The girls simply assumed that I was joking, or having a bad hair day''). And for dishy details of offstage life with Posh, Baby, Sporty, and Scary, British tabloids are far more piquant -- Halliwell's raciest tales involve popping out of too-tight concert costumes. Strip away the prattle-filled chapters on the Spice life, however, and what remains are surprisingly honest bits where Halliwell details her depressive bouts with bulimia and stints as a ''glamour'' (read: nude) model. But it's only reflections like ''singing to Prince Charles is the perfect laxative'' that occasionally keep the memoir from being a yawny wannabe. C+