Sly Stone, who invented his own kind of funky hybrid of rock ‘n’ roll and R&B with his ’60s group Sly and the Family Stone, has been living a private life for decades. Now 80, the reclusive music great has just penned a memoir, “Thank You (Falettinme Be Mice Elf Agin).” Correspondent Kelefa Sanneh talks with Questlove (who published the book), Sly’s longtime friend George Clinton, and with Sly’s real family, about the musician’s revolutionary band; a life revolving around drugs; and how he is today.