How close was this to the book Catch-22?

Yeah, the film was not close enough to fake it if you have an experienced teacher reviewing your essay. I suggest Cliff's/Cole's Notes or some marathion reading.

There's too many drastic differences between the book and the film to pretend one is the other. Different characters die different ways than in the film, Luciana wasn't quite as worked out in the book. Yossarian dated her and there's a relationship of sorts in there, but most of his pursuits were centered Nurse Duckett, some other Italian girls, and an Officer's wife when he was still a Cadet. Nately does have his whore and much of that relationship is fairly verbatim, but his death is much different. There were a lot of characters left out of the film that would be too integral in the book to leave out. Hope this helps.

Yeah, I agree with cwebb44.

It's got a lot similar parts, but to get the real idea you should read the book.