A Greek Grammar of the New Testament (BDF)
Greek grammars typically come in two forms. There are teaching grammars, which are designed to introduce you to the language and offer you a systematic method of learning it. Then there are reference grammars, which offer a systematic and detailed treatment of the Greek language as a whole. These are especially helpful to consult when you run into a Greek expression or construction which is unexpected or difficult. A teaching grammar will tell you that most of the time, the negative particle μή is used with a verb in the subjunctive. A reference grammar will tell you why it is occasionally used with a future indicative and whether or not that change is exegetically significant.
Among Greek reference grammars, The Greek Grammar of the New Testament and Other Early Christian Literature by Blass, Debrunner, and Funk (affectionately known as BDF) is a recognized standard intended to complement A Greek-English Lexicon of the New Testament and other Early Christian Literature (otherwise known as BDAG). BDF is distinct among grammars of New Testament Greek in that it "sets the Greek of the New Testament in the context of Hellenistic Greek and compares and contrasts it with the classical norms." That's especially helpful to folks like me who took classical Greek before learning Koine. "Designed to compress the greatest amount of information into the smallest amount of space consistent with clarity," BDF is quite dense and includes "subsections discussing difficult or disputed points." For scholars, students, and pastors doing serious work in New Testament Greek, BDF is a must-have resource.