It is not. The end.
I don't know how many other ways I can explain this: the hobbit is not included in the "LotR" grouping because the plot is different, but it is part of the larger informal "LotR" story grouping because it precedes the other books. If I were to publish two books, one a sequel to the other, but not officially label them as a series; they're still a series because of the definition of the word "series":
A number of things, events, or people of a similar kind or related nature coming one after another: "a series of lectures".
according to what?
http://acronyms.thefreedictionary.com/LOTR
The series is called "The Lord of the Rings," not "The Lord Of The Rings."
http://en.wikipedia.org/wiki/LotR links to Lord of the Rings, but
http://en.wikipedia.org/wiki/LOTR merely suggests a connection.