Commentaries are an important tool for any scholar. They serve as an introduction to many of the critical scholarly discussions concerning the New Testament book in question as well as provide important bibliographic information for those that desire to go more in-depth than what many commentaries do.
The commentaries below are, for the most part, pre-20th century; however, that does not diminish many of the contributions they have made to biblical scholarship. As with most things, many of these commentaries are "out-of-date" in many respects but that does not dismiss the important information provided.
Below are links to individual pages for each of the books of the New Testament wherein commentaries will be found that focus on that one book (though sometimes authors will combine the smaller books).
General introduction commentaries are just that; they focus a limited amount of space to each of the books in the New Testament. These are excellent because they focus on the most important aspects of the books that are generally agreed upon by most scholars and are suitable for general audiences.