I bought a copy of “AC Power Interference Handbook” by Marv Loftness (KB7KK). While written by a professional for professionals, this book is an excellent and easy to understand resource for the rest of us. I highly recommend it to anyone having power line noise problems. The book is available from ARRL and other book sellers.
Another excellent resource is “The Mitigation of Radio Noise From External Sources at Radio Receiving Sites”. This is from Naval Postgraduate School and is in the public domain. As far as I know it is only available as a PDF file. You can download it here (6 MB file size).
The ARRL web site has an extensive section on power line noise.