Microsoft recently restricted the amount of apps developers can submit to the Marketplace to 10 per-day. This was in response to a continued injection of spam, some apps being completely useless. As one can see in the above video, captured by SolidSnakeSK, ScrollMotion Inc. has been able to submit more than 10 apps (or books in this case) to the Marketplace, which were approved. While this isn't spam as such, it does prove to be an issue with scrolling through "new" apps etc. and one can only assume that certain developers can bypass the restrictions depending on what they're submitting. 

It would make sense for one app to be used as a gateway (or digital book shelf) where books can be purchased and stored inside the app itself. Having books published separately will clog up the listings, but will increase total Marketplace numbers of course.

Source: YouTube, thanks Snake for the video!