...AND JERRY MATHERS AS "THE BEAVER"

...AND JERRY MATHERS AS ''THE BEAVER'' Jerry Mathers, with Herb Fagen (Berkley Boulevard, $13) Finally, the sordid, behind-the-scenes truth about Leave It to Beaver. This tell-all from the Beav himself gives you the skinny on Barbara Billingsley (June Cleaver): She's ''a person whom everyone unreservedly likes''! And the pressures of show business: Mathers had to wake up at 6:30 every morning! And the eyebrow-raising tale of Ken Osmond (rascally Eddie Haskell): There's a rumor that he grew up to become porn star John Holmes. Too bad it's not true. This revelation-free memoir -- in which everyone gets tagged with the adjective ''nice'' -- could desperately use some excitement. As it stands, it's about as riveting as mowing the lawn. We learn that Mathers grew up to become a banker, dinner-theater actor, and proud NRA member. Good for him for avoiding the traditional post-child-stardom descent into petty theft and heroin addiction. But jeez, do we really have to read about it? C-