I've read a couple of books by Sinclair B. Ferguson- this one, and his "Lets Study" guide to Mark's Gospel. When looking at both the Sermon on the Mount, and Mark's Gospel, Ferguson's weren't the only books I read. But in terms of helpfulness-per-word, they were both at or near the top of the list. When you read a stack of other commentaries on Matthew and the Sermon on the Mount, and do a reasonable amount of thinking, it becomes obvious that although Ferguson wears his scholarship lightly, it goes deep. This book is a popular-level treatment, and it's easy to read and follow. But it demonstrates a grasp of what the sermon is supposed to be about and how it fits into the broad historical purpose of God far better than books which weigh ten times as much.
To analyse the words of Jesus here as Mr Ferguson has done is brilliant, it brings out the meaning of what Jesus was trying to say. Foreign language, translations, different culture mean that it is difficult to get at this meaning.