Legal Question in Intellectual Property in California

There is some misinformation about copyright and registration floating around the web, written by people who are not copyright lawyers and don't know the law. Your best source of information is the copyright office itself, which offers free online publications about the finer points of registration. To answer your questions:

Copyright attaches to a work when it is created, regardless of whether the work is registered. Registration is a legal means of claiming, and proving in court, that you created a work. Books written after 1978, in general, have only a single copyright date. An exception would be if the book is a collection of short works written at different times. Perhaps that is why you see multiple dates in a single book. Or, if the book was registered before 1978, a copyright renewal was required. That might also account for more than one copyright date. (If neither of those scenarios applies, it is likely that the person placing the copyright notice simply didn't know the law and wrongly supposed they were gaining some advantage.)

No, you can't extend the term of copyright by re-registering a work. Copyright is determined from creation or first registration, unless an unpublished work is later registered as a published work (which may change the calculation of term of copyright). If you make revisions, and want to copyright the revised book, the second copyright would attach ONLY to the actual revisions, not the entire book. On the registration form, you would be asked if a prior work existed and you would indicate that you are applying for copyright for the revised portions only (and only for the actual words added).

Re-registering an already registered book would probably be a waste of time and money. Only the copyright holder has the legal right to make changes or revisions to the copyrighted work, so it really wouldn't give you any additional protection. You would still be able to prosecute any infringement of your work.