Most talk of Frege is concerned with his work on sense and reference (1892), but this is a later paper (1918) in which he clarifies some ideas that inform his theory that led to that original paper.
A good amount of our research agendas concerning presupposition stem from interpretations of his 1892 work. Lauri Karttunen brought this to my attention in "Presuppositions: What went wrong?" and since I'm writing a paper on Karttunen and presupposition, I figured I'd better read this. He also mentioned that we can use this 1918 paper of Frege to fix some of our misconceptions about presuppositions.