While not an expert, I have read all of the Studies (quite a few years ago, I must say) I can only say that I think that the Divine Plan of the Ages should be required reading for any who want to understand the JW religion. The really juicy things that most people refer to (pyramid, time prophecies, etc) are in the later volumes. The main thing that comes thru in the Divine Plan is that:
1 Russell was a good writer, The invention of the JW religion wouldn't have happened if Russell had not had a good command of the English language, and he was good. Compare the Divine Plan to the Three Worlds, which was written mostly by Barbour and financed by Russell, and there is no comparison. It is an easy read, even a hundred+ years later.
2 The Divine Plan is all about going to heaven, for the 'elect' ie; the best Christians. The earth was only for the second best Christians, and the 'heathens', and for the 'Jews' who were given a better place than the heathens, but still on earth. It is all ridiculous now of course,, but one cannot really get what happened in the 20's and 30's without a background in the Divine Plan.
Incidentally, the Divine Plan, as well as the Studies and some of the other pre-1916 literature, is usually in print by some of the Bible Students Groups. The WT society continued to publish ( or arrange to be published) the Studies into the 20's, but the edition that the Bible Students reprint is (correct me if I am wrong) the 1915 version. The differences are minimal.