If you passed over The Beatles when this authorized biography’s first edition appeared in 1968, be advised that author Hunter Davies’s account of the group’s career remains definitive and highly recommended. If, on the other hand, you read the book “way back when,” you needn’t bother with this so-called “revised edition,” which adds almost nothing to the original. Davies does include a short new introduction, where he admits to “very little interest” in the Beatles’ post-group activities; the only other notable addition, a brief postscript on the solo years, underscores that admission with a perfunctory discussion of facts any fan already knows.