Music Publishsing is one of those things that is somewhat akin to quantum physics and credible politicians - you're pretty sure it exists, but most can't explain it or give a good example of how it's used.
Making Music Make Money (Berklee Press, 2004) by Eric Beall does exactly that...not the phsyics or politician thing, the music publishing thing. Too often music business books set out to cover as much as possible and this subject, as important as it is is usually completely glossed over.
Eric Beall is the Vice President, Creative at Sony/ATV Music. He was the former Creative Director for Zomba Music Publishing. Ergo, he knows his stuff to say the least. He discusses the innards of publishing from the stand point of someone starting a publishing business from the ground up. He covers topics such as copyrighting, building a business plan, reviewing material, building relationships, and how to sell your songs. This book is bar none, the most thorough piece of material I've seen to date on the subject.
For those musicians that are planning on picking up a copy of This Music Business, I still recommend on doing so, but when you want to learn about publishing skip the five pages in the big book and use Making Music Make Money as your publishing how to.