Which book by Nagarjuna provides an introduction to his teachings? I've noticed there are a lot of YouTube video lectures on Nagarjuna. It seems he was a very prolific writer on Mahāyāna Buddhist philosophy. Which of his works would be appropriate to get an introduction to his expositions on śūnyatā, the two truths, or relativity?
The word "introduction" may be out of place, but anyway his most important work would be the Mūlamadhyamakakārikā. A translation, along with an extensive commentary, is available in The Fundamental Wisdom of the Middle Way by Jay Garfield (here is a previously published commentary of chapters 1, 2 and 24). Bear in mind that this is a very, very difficult text to understand, even with the (indubitably necessary) help of commentators. Toward that end, let me point out Nāgārjuna's Madhyamaka: A Philosophical Introduction by Jan Westerhoff, which provides a synoptic overview of the arguments in Nāgārjuna's corpus concerning different philosophical problems with the aim of presenting an account of the whole of his philosophy (here is a helpful review of the book).