This past summer, I was delighted to discover a new “ethnic detective” character named Masuo “Mas” Arai, an elderly Japanese American gardener whose credentials include a green thumb and a nose for sniffing out criminals. The creation of Los Angeles-based journalist and author Naomi Hirahara, Arai made his literary debut in 2004 in “Summer of the BIG BACHI.” A year later he was assisting the NYPD in “Gasa-Gasa Girl.” This year, he's back cruising the L.A. freeways in his battered pickup truck in “Snakeskin Shamisen,” where a traditional Okinawan musical instrument left at the scene of the crime led him to the killer.