3. Kazan’s sense of feeling aimless and artistically spent was rare for this relentless achiever, but certainly understandable. What does artistically spent mean?
In this case, "spent" means exhausted or used up. So "artistically spent" means that he feels artistically exhausted or empty. He feels unable to be creative.
4. But by this juncture Kazan had already amassed a momentous body of work in theater and film that testifies to his unstoppable drive and restless energy. What does “amassed a momentous body” mean? Does it mean he is sick?
> A "body of work" is a collection of "works" (books, poetry, essays, paintings, sculpture, music compositions, etc.). A person's body of work may consist of only one of these categories or, if they are multi-talented", it could consist of more than one category (e.g. both novels and poetry or both paintings and sculpture).
The word "momentous" seems to be misused here. The definition of "momentous" is "(of a decision, event, or change) of great importance or significance, especially in its bearing on the future." A better word to use to describe a large "body of work" would be "monumental". The sentence is trying to say that the large number of works that he has produced are evidence of "his unstoppable drive and restless energy".