Chelsea Clinton may be, like her mother, a little hard to pin down as a personality, but three things seem inarguable after reading that email: She has a big heart, she writes an excellent policy paper and she is verbose. The voice you hear there addressing “Mom” and “Dad” — both wonky and sweet, impassioned and didactic, a little self-conscious — is pretty much the same one we hear in her new book for children, “It’s Your World.” It’s clear that she indeed wrote it herself, all 400 or so earnest, methodically compiled pages. The book is meant to help...