Complex sentence: In the early 1930s Bemelmans met May Massee, the children's book editor at Viking Press who became a sort of partner, and he began to publish a number of children's books beginning with "Hansi" in 1934.
Split into simpler sentences: In the early 1930s Bemelmans met May Massee, the children's book editor at Viking Press who became a sort of partner. He began to publish children's books, beginning with "Hansi" in 1934.