I recently finished reading "Gentile Times Reconsidered" by Carl Olof Jonsson (GTR by COJ).
The book focuses on shedding light on the 607 date, but there is a good deal of history on the early Adventist movements, as well. The book is well written, easy to read but is very detail oriented and contains significant technical jargon.
CyrusThePersian's idea is a good one; this steers you out of the "apostate" arena. You will have to stress: these sources NEVER WERE OF JW ORIGIN, so they can't be "apostate".
However, these sources my be very scattered. So, you will have a tough time finding a "non-apostate" reference that is more helpful than GTR. That is because no one but an "apostate" would go to the effort to concentrate so on the 607 date.
However, you can gather the sources and references used in GTR into a list and say "check these out" to someone. Or you can gather a bunch of these and present them "standalone" w/o GTR.
If anyone wants to challenge the ideas in GTR and champion the 607 date, knock yourself out. My money is on COJ finding another counter-argument.
The following are my initial personal notes, written to refer to when I read it the second time, so as to make a comprehensive review of the book:
1) The Bible has no astronomical references for use as a "touchstone" w/ an absolute date. Thus the Bible is only a RELATIVE CHRONOLOGY; Babylonian and other records that refer to astronomical data are ABSOLUTE CHRONOLOGIES.
2) Judean Regnal Years differ from Judean Calendar Years (Tishri start versus Nisan start).
3) Regnal years differ for Judah & Babylon (accession versus non-accession years).
4) WTS applied 70 years (Jeremiah) wrongly: the destruction and subsequent desolation of Jerusalem was only 50 years. The case is made that Jeremiah's writing actually referred to "70 years for Babylon"; this "70 years for Babylon" is SERVITUDE to Babylon by all or most of the nations in the Middle East.
5) Once common dates (where the Bible refers to Babylonian history and Babylonian history connects to astronomical reference points) are applied to #1, and #2 & #3 are factored in, the Bible and Babylonian records combine and agree very well w/ secular history (where secular history is defined as that taught or based on sources beyond the Bible).
6) The Bible, Babylonian records & secular history agree on Jerusalem's destruction in 587. BTW, Babylonian history and records are probably the most well established of all ancient history prior to Christ.
7) Only WTS's misapplication of 70 years creates 607 BC as the date for Jerusalem's destruction. No other supporters of this date are to be found. Note that the case is made that WTS actually disagrees with the Bible, as well as secular history and Babylonian records.
8) #7 ALLOWS FOR 1914 AD (w/ other minor mod's); this date seems to be arrived at w/ the sheerest of luck and probably a great lack of information at the time (~1870's). 1914 hit a major world event & has seemed significant enough to be the "cornerstone of something".
9) Due to #8, WTS cannot back down.
10) The GTR book points out 14 "lines of evidence" which DO NOT REQUIRE FAITH ALONE against the 607 date; WTS has only one very weak "line of evidence", and thus drops back to a requirement of FAITH against credible scholastic evidence. The one weak "line of evidence" is very nearly scholastically discredited.
11) The WTS theories have almost no scholastic evidence for support. The WTS theories require FAITH IN THE PROPOSED THEORY for their only real support, in the face of overwhelming scholastic evidence to the contrary. In addition, it is hoped that "new and yet undiscovered documents or manuscripts will be found to reverse the current understanding of secular history".
12) The last statement of #12 points out the summation of WTS Chronology: WISHFUL THINKING!!!
Mustang