Where Britney Spears's first two albums hewed to the early-'60s formula of one strong single plus a stack of filler--a fair bit of which ended up on the radio anyway--her third justifies itself as a full-length listen. Led by the single "I'm a Slave 4 U," a Neptunes-helmed piece of electrofunk that promises she'll do anything you want as long as it's dancing, the album continues through superior versions of Spears's poses. Calculated frustration with the adult world? Calculated independence? Sheer celebration? Check, check, and check: "Overprotected," "Let Me Be," and a cover of "I Love Rock & Roll" that brings to mind its bubblegum roots. Even without the joyous disco tribute of "Anticipating" and the not-icky ballad "I'm Not a Girl, Not Yet a Woman" providing symbolic respite from the rest of this machine-tooled music, Britney is one of the most human discs of the current teen-pop boom. If it spins off the deserved string of radio/video smashes