I've been reading through Motorcycle Electrical Systems Troubleshooting and Repair", ISBN 978-0-7603-2716-6. Fantastic book, which explains how to troubleshoot electrical systems, to avoid spending money on replacing parts that are not at fault. Surprisingly, quite often the issue is a bad connection, or wire. He explains it all quite well.
One caveat: I was trained as an Electronic Technician, having worked as a component level repair technician for over three years. I skipped most of his explanation of how electricity works, but I skimmed it: rather a good, simple, understandable explanation. I would highly recommend this book. He shows how to check for voltage drops with a basic, $20, Digital Volt Meter. A high current amp probe is recommended, but it is not always needed. Best of luck. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII