You can buy an OEM wiring manual by calling 800-25-volvo for your car, not a bad thing to have for any car really.

I've seen loose starter 'S' wire plugs (smaller single plug wire that triggers solenoid) at the starter solenoid on a few newer cars so definately start there as that wire is sometimes pulled too tight, not enough slack. Remove the plug to see if it's loose fitting, easy fix.

Otherwise, "could be's" include starter relay in engine fuse box (can bypass relay jumping terminals 30 to 87 which will illiminate the relay if you can duplicate the no start), otherwise check for signal ground and ign switched 12v+ at the relay plug from the ign switch. '98s were known to have bad ignition switches as there was a recall on them. A bad PNP switch can't be ruled out but it's be VERY rare on a newer car with low mileage like yours (assuming?).