It's a shame to have to point out such underhanded behavior on a holiday weekend, but we got a heads-up from developers Alexandru Brie and Patrick Thomson that something was seriously amiss in the Books category on the App Store. As detailed on Alex's blog and in this follow-up from The Next Web, it appears that at least one Vietnamese developer (mycompany/Thuat Nguyen) has gamed the category so that 40 of the top 50 entries are all apps from their stable. It's possible that another two or three developers have tried the same thing, although to a less extraordinary degree.
As Alex points out, at least two of the reviews for these apps indicate that the apps were purchased against the buyer's will, with hacked iTunes account credentials. The ongoing MacRumors forum thread here indicates a possible spike in iTunes fraud, with major purchases being charged against unsuspecting users' accounts. Alex also notes that Apple's response team is already looking into the issue.
If you have unexpected charges on your iTunes account, be sure to notify your card issuer or bank immediately, and then send word to Apple. It may take a bit longer to get help during the long weekend.
Updates: 9to5Mac points out that the offending apps have been pulled from the store. Arnold Kim at MacRumors reminds us that compromised iTunes accounts have been reported for years, and it's not clear that the current circumstances represent a spike in malicious activity.