I only recently started the book, so forgive me if I'm missing something that's made obvious later in the book, but does the title of The Neverending Story have a double meaning? Bastian assumes when he starts reading that it just means that you can keep reading the book as long as you want, but does it mean that you might literally be unable to stop reading it or "get out of" the story? That would technically be a "neverending story."
As I mentioned in a comment, answering this question basically involves spoiling most of the plot - presumably you've long since finished reading the book, so I mention this for the sake of anyone else reading this post. Basically the answer is yes, although some closely related issues, on which follow-up questions could be asked, are possibly more interesting and reliant on deeper readings of the book.
When he takes the book and starts reading it, Bastian assumes the title means it's a story that he can keep reading forever. We've all heard about gripping stories that "pull us in" as readers, but Bastian could hardly have expected that this story would literally pull him in, that he'd become a key part of the world it describes, and even responsible for rebuilding that world from his own imagination. That's how it's really a Neverending Story: the reader gets pulled into the world of the story, becomes a part of it.
In Bastian's case, one might say that the story does end, just like the book that you and I read: he comes out of Fantastica and resumes his life outside of the story. But even the story that we read is not completely finished - there are many loose strands which could be picked up to weave entire new stories. Perhaps, for a sufficiently motivated and creative reader, the book that we read is itself a Neverending Story that could suck us in to finishing those unfinished parts of the story, contributing to creating some parts of Fantastica ourselves, in a sort of metaphorical version of Bastian's literal journey into the world of Fantastica. Also, on a darker note, many of the people like Bastian inside the story - people from our world who came to Fantastica by entering the Neverending Story - really do find it neverending: they reach the City of Old Emperors and never manage to escape from Fantastica, losing their memories and becoming wrecks.
So yes, the title of the book means more than Bastian realises at the beginning. The book he's reading has the power to draw him physically (or metaphysically?) into another universe, and the potential to leave him trapped and insane in that universe forever. Perhaps the same title of the book that we read means more than we realised too ...