Lionel Bascom, a writing instructor at Western Connecticut State University, has a thing about Harlem. He spent part of his childhood there and as a scholar he has studied it ever since. His latest book, “Harlem: The Crucible of Modern African American Culture,” shows that not only did the flowering of African-American literature and music endure in Harlem much longer than is widely documented, but much of black culture in the United States was forged there.