Peter Boston (1918 - 1999) was the son of Lucy M Boston and illustrated her books, he was also an Architect (an obituary here gives more information about his architectural career). This long but interesting interview with Diana Boston (Peter's wife) who does a sterling job of mainining the house and showing people round it includes a section on how Peter did his illustration. If you want just that bit it is in the section entitled "Architecturally Speaking".