Interesting point FJL. I certainly defer to your legal expertise. Though there does seem room for complaint when one goes by the USPS definition of mail fraud:
MAIL FRAUD - 18 U.S.C. 1341, makes it a Federal crime or offense for anyone to use the United States mails in carrying out a scheme to defraud.
"A person can be found guilty of that offense only if all of the following facts are proved: First: That the person knowingly and willfully devised a scheme to defraud, OR FOR OBTAINING MONEY OR PROPERTY BY MEANS OF FALSE PRETENSES, REPRESENTATIONS OR PROMISES; and Second: That the person used the United States Postal Service by mailing, or by causing to be mailed, some matter or thing for the purpose of executing the scheme to defraud."
The misrepresntations and promises seem clear enough. The "knowingly and willfully" would be harder to prove, of course, as the guy would just argue that things kept going wrong out of his control. However five years without a book seems somewhat willfull.
The ideal would be to find everyone who ordered and get them to simultaneously ask for their money back, then see what happens.