I decided to bite the bullet and spend several hundred dollars on some factory manuals.
One confusing item was the wiring diagram. http://www.volvotechinfo.com lists two volumes of wiring diagram for 1999. One says "for late models". I emailed them asking what was the VIN cutoff for where "late models" began, but they couldn't tell me and referred me to "an authorized Volvo dealer". Nobody at the dealer could tell me either. Their advice was "buy both". Easy to say when you're spending $50 of someone else money.

My car was built in late March 1999, and is probably a late model. Does anybody out there know what was the cutoff VIN? Any idea what would be different between early and late 1999? (BTW, the 2000 models have yet another, different wiring diagram. A lot of changes for what seem like identical vehicles.)

Believe it or not, even Volvo techs are not 100% sure of that VIN cutoff between early vs late models of the same year as Volvo doesn't make it THAT specific. Usually, the manuals are similar and the late model book is often nothing more than a suppliment to the early model with details on devices or changes that were made mid-year.
It being your money and that you're probably right in assuming that your car is a late '99 model, I'd suggest starting with the later book and see if it's good enough for what you need it for. If it then seems more like a suppliment (not complete), then order the earlier one too.