The book is based off of LuSid's design, and yes it doesn't have a back.  A friend of mine built a cabinet with that design, and it has no back either.  However, when it's turned to a wall or corner, it's not noticeable there isn't one there.   You certainly could add a door on the back.  For some people there is no need for that.  For others, it's possible to add one for "authenticness", "security from the kids", or whatever.