(1) Talk to your professors, who will have a better sense of what is appropriate for you at this point. They will be better able to give you specific suggestions.
(2) You're going to need a lot more tools in your toolbox to do research eventually, so while your enthusiasm for research is great and I certainly encourage you to dabble in any questions that come up along your way, you are going to need to take lots of coursework in the future as well. At this point in your development, doing coursework on your own -- going through the additional material in the textbooks you have as 3Sphere mentioned, as well as reading one or two appropriately chosen, more advanced textbooks (here, specific advice from your profs can be useful to select texts that match your current knowledge base) -- is likely going to be the best way forward.
(3) Given your background, you might be ready for an introductory undergraduate text on Fourier analysis. You could also read a text in a new area, such as algebra, topology, or combinatorics.
(4) If you are interested in combinatorics, Kenneth Bogart made freely available a self-learning textbook that he was working on when he sadly passed away. It is available here.