As pointed out in Emilio Pisanty's answer, the main question was not actually asking for a recommendation for a source from which to learn about a topic. Rather, it was asking about the usefulness of one commonly used resource (Jackson's Classical Electrodynamics). Personally, I think it would have made sense to delete the tag, rather than make the question a community wiki—although this might be seen as conflicting with the original questioner's intent. There is a further question of whether such questions can be on topic at all; the question about Jackson is currently closed, although it does have three reopen votes.
How do we think similar questions should be handled in the future?