ROCKPORT – When children’s book illustrator Melissa Sweet begins a new biography, she wants to know what her subject was like long before they achieved the things that made them noteworthy. “What is the link between what we know of this person today and what they were like as children? Most of the time, there is a link,” she said. Sweet found an obsessive little boy when she researched “The Right Word: Roget and his Thesaurus.” Peter Mark Roget’s father died young, and little Peter threw himself into his books.