Someone else reported this a few months ago.  When BookMacster's built-in app updater finds a new version of BookMacster and downloads it, it temporarily parks it in that Application Support folder.  I surmise that, if something gets interrupted during the update process, the update might get orphaned there.  I haven't investigated any further, but you are correct that you should trash them.

Like most Mac apps distributed outside the Mac App Store, BookMacster uses the open sourced Sparkle Updater for this function.  So you might see it in other apps too.