Jan Brett creates some of the most intricately drawn children’s books around. Her stories, often populated by woodland critters of many varieties, have delighted young readers for years. Her books such as “The Mitten,” “The Hat” and “Mossy” have sold more than 36 million copies. Brett’s latest book, “Animals’ Santa,” was actually born during the heat of summer. Her husband, Joe Hearne, is a bassist with the Boston Symphony Orchestra, which every summer does a series of performances at Tanglewood, in the Berkshire Mountains in western Massachusetts. While staying at their lake cabin at the hottest time of summer, the idea of a Santa for animals just popped into her head, she said.