I had been planning on asking three professors to write my letters and, while two have been eager to help, the third professor has been completely unresponsive to my emails. I'm now working on finding a third recommender to ask instead. The person who knows me and my research the best is definitely a grad student that TAed the class in which I wrote the paper. He's a great guy, ABD, and was insanely enthusiastic about my paper, but I'm not sure how to go about this. Should I just ask him? Should I have him write the letter, but ask the actually prof (whom I barely know) to actually sign and submit it? If so, how do I go about suggesting that? I'd be insanely grateful for any thoughts or suggestions.