Complex sentence: While at Yale Landesman got to know novelist Jerzy Kosinski and he worked with Kosinski on his novels Being There and The Devil Tree as an editor, to help Kosinski, not a native English speaker, with his syntax and writing.
Split into simpler sentences: While at Yale Landesman got to know novelist Jerzy Kosinski and he worked with Kosinski on two of his novels, Being There and The Devil Tree. Landesman was involved as an editor, helping Kosinski, not a native speaker of English, with his English syntax and writing.