Register for a copyright through the U.S. Copyright Office. Although there is an implied copyright imparted on anyone who publishes self-created content online, the reality is that, without an official copyright from the US government, you would have no real basis for defense were a copyright infringement case to arise. Therefore, it is an absolute must that you procure an official copyright. The required form takes only a few minutes to fill out, it costs $45 and it will be processed within four months.
Print out a copy of your e-book, save it to a CD-ROM and send it to yourself through official US Post Office mail. Do this after receiving your copyright, but before displaying any of your content online or in print. This provides verifiable proof that the e-book was in your possession before it went online, and can be used both to strengthen cease-and-desist orders sent to potential plagiarizers, and to defend your case in any ensuing court battles.
Post a copyright statement at the bottom of every page of your e-book. Although plagiarizers may not be deterred, it will remind honest people that this content can't be copied. To further bolster your defenses, make sure you write out the word "copyright," as opposed to simply placing a "(c)," or "c," at the bottom or your page. This is important if you plan to make your content available in countries outside of the United States, where such symbols may not be legally recognized.
Obtain an International Standard Book Number, or ISBN. This step is optional, being that it is both costly and not technically a legal asset. Although an ISBN number can't necessarily offer a greater degree of legal protection, it can get you registered in the Books in Print database, which serves to legitimize your book as a commercially circulated publication, bolster public records of your copyright for court cases and give you greater exposure to book retailers. This process can be conducted at isbn.org, available in the resources section. It takes 10 days to process and costs $225.
Consider getting an EAN barcode. This is a must if you plan on circulating your e-book via CD-ROM. It's identical to the barcode on printed books and instantly sources your CD-based e-book to it's ISBN registration, making sure that each physical copy of your book is linked to you name, your ISBN number and your copyright information.
Sign-up with a digital content protection system. Much cheaper than the ISBN and EAN options, this system is specifically designed for e-books. Instead of just bolstering your copyright credentials in anticipation of a plagiarism case, this system seeks to prevent plagiarism altogether by protecting your e-book at its source, and making it impossible for plagiarizers to copy your work. If your interested in this option, check out digital-cp.com or locklizard.com, two popular content protection systems.