One WT book that states the reasons why the WT believes the 144,000 and great crowd are two different groups, with the latter having an earthly hope, is the book called "Then Is Finished The Mystery Of God". It is copyright 1969. It does not likely completely agree with the current WT teaching on the matter, but at least it will tell inquiring people what the WT taught on the matter from 1969 to around 1980. It encourages readers to read some specific WT books and articles from before the year 1935 and maybe that is part of the reason the book is not included in the WT Library CD-ROM, nor the WT's online library. A transcription of the book (apparently by a faithful JW) can be read online at http://www.strictlygenteel.co.uk/mystery/mysterytitles.html . I noticed there are some minor typographical errors in the transcription of the book, but that source is still very reliable. That source for the book includes the illustrations of the book.
During all of the years I was a JW I had very little to no interest in the book. It bored me immensely (and I think I had a very hard time understanding it when I was under the age of 10 and even as a teenager), but during the past 7 days I have been reading the book with interest. During the past seven days I have become increasingly fascinated by the book. Now I understand the WT's perspective and reasoning upon the chapters of Revelation that the book discusses, and as a result now I can see how many JWs could believe the WT's teachings on the subject. I am now impressed by how skillful the author of that book was stating his line of reasoning for the WT's doctrines pertain to the prophecy of the book of Revelation. However, despite that I disagree with a large number of the WT book's teachings. I am convinced that according to the book of Revelation, the great crowd during the 1,000 years are in heaven and serve in the temple building. That realization is one idea which caused me to stop believing that the JW religion is the one true Christian denomination of Christianity.
I once or twice owned a hard copy of the book and of the Babylon the Great book, but I later sold them because at the time I was unimpressed with them and I wanted to downsize my personal library of WT books. But, now I regret selling those two books. My interest in the books is not so such for what bits of truth they contain (and there are some truths in them, including some historical elements and including some correct interpretations of what various biblical passages are intended to mean), but for the insight they give me of the WT's reasoning in support of their doctrines which existed during the time from my preteen years of life through my years as a college student. Now I even find reading the "Then Is Finished The Mystery Of God" as entertaining, like reading a fiction novel.
The following is something which puzzles me. If Revelation teaches that the great crowd who comes out of the great tribulation will be in heaven, and even more so if it says they are the same group as the 144,000, then according to Revelation who will Christ and his co-rulers be ruling over? I don't think Revelation says they will be ruling over themselves (like in a pure democracy) for Revelation says they will rule over/upon the Earth. But, according to Revelation who will be on Earth (to be ruled over) during that time if the great crowd and the 144,000 are up in heaven? One chapter in Revelation says Satan will be in the abyss for 1,000 years so he can't mislead the nations any more, but an earlier chapter says that all the people who receive the mark of the beast will be slaughtered by the god's execution forces and it says that the beast will slaughter those who refuse to receive its mark. If someone knows (or thinks s/he knows) the answers to these questions, please provide those answers.