The only original song lyrics in the movie are those to "The Greatest Adventure" and other sections where that melody is used. All others are directly from the book as written by J.R.R. Tolkien or adaptations of what he wrote in verse form.
In the book, Bilbo is knocked unconscious by a falling rock during the Battle of Five Armies immediately after seeing the eagles arrive to help. In the movie, perhaps due to an anti-war bias at the time of filming, Bilbo states that he "simply doesn't understand war" and then hides behind a stone, using the ring to become invisible and watching the entire battle. When asked about his whereabouts Bilbo lies and says that he had "a bump to the head" and was "out for hours".
The Arkenstone, an heirloom of Thorin's dynasty and the most important treasure that he wishes to recover, isn't included in this film so neither is the surrounding storyline of Bilbo stealing the Arkenstone and giving it to the elves. This event causes noticeable friction between the two of them in the book which is explained in the movie as Thorin's irritation at Bilbo's lack of understanding for war, something that doesn't happen at all in the book.
The song sung while Bilbo and the dwarfs as they walk through Mirkwood, "Roads go ever ever on, Over rock and under tree, By caves where never sun has shown, By streams that never find the see," is taken directly from the book, although in it Bilbo sings it as he first sees Hobbitton upon his return.
Gandalf at the end says that members of Bilbo's family not yet born may one day understand the ring. And that the story is not really ending, but just beginning. These lines were not in the book, nor was there any indication that there was any more to the ring than mere invisibility. At the time the original book was written, there was no intention to connect The Hobbit with the Tolkien Legendrum which includes The Silmarillion and The Lord of The Rings (which hadn't been written yet). However, in 1977, it was well known that the ring Bilbo found and the events he participated in were a prelude to a much larger story.