I finished the book this morning. It is, without a doubt, one of the best books I've ever read. I whole-heartedly encourage you to buy a copy and read it. It's amazing.
Now, the fact that it took me more than two years to read is suggestive. Some of that is because I was reading a lot of other things at the time. Some of it is that I tend to do everything inconsistently, especially reading: it's not at all uncommon for me to read several hundred pages of a book, then put it down for months or even years before picking it up again (I've got bookmarks in at least a half-dozen books at any given time). But in this case, I found the book bogged down significantly in the middle. It seemed to get a little dry, and it took months for me to work through it, sometimes no more than a page in a day. But once I got to around page 300, I found I had trouble putting it down.
I've been interested in the Biblical teaching about the Kingdom for many years. I spend a lot of time around Dispensationalists, who seem to fear the word "kingdom." But at some point I realized my reticence to study the Kingdom was not based on Scripture. The Scripture spends a good deal of time discussing the Kingdom, indeed the book of Acts ends with this description of Paul's ministry:
So certainly the Apostle considered the Kingdom of some importance.30 ¶ And he remained two whole years in his own hired lodging, and received all who came to him,31 preaching the kingdom of God, and teaching the things concerning the Lord Jesus Christ, with all freedom unhinderedly. (Acts 28:30 & 31, JND)
What I love about this book is, it studies the Kingdom from a dispensational perspective. While unapologetically premillenialist and dispensationalist, it's also a passionate and enthusiastic study into Christ as King. McClain doesn't shy away from the place the Church has as part of the Kingdom: indeed, he almost exults in it.
McClain's book is a study of the Kingdom from Genesis to Revelation. It's written as a text-book, and it's very matter-of-fact. That's probably why I found the middle of it quite dry. But it's not a boring book: not by any means.
What I found most amazing in the book was McClain's study of the four Gospels in relation to the Kingdom. There's no doubt that the Kingdom was the main focus of the Lord Jesus' earthly ministry, and McClain expounds it brilliantly and clearly. His discussions of the Triumphal Entry and the Olivet Discourse are well worth the time, effort, and expense of getting a copy and reading it through.
I've now read three of McClain's books: The Greatness of the Kingdom, Law and Grace, and Daniel's Prophecy of the 70 Weeks. The Greatness is by far the longest book I've read by McClain: Law and Grace is a very short book, while Daniel's Prophecy is little more than a pamphlet. Nevertheless, I've become a real fan of McClain's. He does an excellent job of arguing simply and carefully from Scripture, without becoming too abstract or theoretical.
If you're going to read The Greatness of the Kingdom (and you should), I recommend you check out either his book on Daniel's 70 weeks, or even The Coming Prince by Sir Robert Anderson first. An understanding of Daniel 9 will be helpful to McClain's treatment of the Lord's earthly ministry.
I made the comment quite some time ago that McClain is kind of like Darby, but a lot easier to read. I meant that as a compliment. But where Law and Grace is almost a précis of JND's teachings on the subject, and Daniel's Prophecy of the 70 Weeks is a sort of summation of The Coming Prince, I haven't read anything that goes into this of depth on the Kingdom of God. The book is a masterpiece, and everyone should read it.