Maybe not strictly a YA book but possibly is... young kid who's really, really into UFOs & flying saucers... he gets a picture (I think) of a saucer, which at the climax of the book turns out to be a hoax - BUT in the final few pages he actually sees a real UFO! I got this out of the library a few times when I was younger (book would be pre-1978 or so) and can't remember anything else about it. Any help appreciated!