Which book by Nagarjuna provides an introduction to his teachings?
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).