I volunteer at an observatory for a local amateur astronomers' society and one of the guests at a recent star party came up and asked, "What do you think is the most important question science has to answer right now?" Obviously, there is no right or wrong answer, but after the party was over a lot of us were still talking about this question and I ended up learning a great deal from my fellow club members that I might otherwise not have.
The next time this question gets asked I want to be prepared to offer a variety of answers from differing fields and opinions. I don't expect to represent every answer as an expert, but I'd like to be able to give a few more examples than I was able to, and then correlate them to some book recommendations from the answers in this thread about introductions to your field
I also think it is important to frame the question in a way that can be meaningfully answered, i.e. "What is the most important scientific discovery about to be made?" or something like that.
Of course, I had my own answer in mind, but as a relative layperson to that branch of study I had a really hard time articulating why
it was so important to "science." Therefore, if you are uniquely affiliated with a specific field that you think will produce a game-changer, feel free to get as technical as you're comfortable doing.