The free versions are the old translations which were extensively abridged and edited by their translators. While the Gutenberg text remains a great tale of ingenuity and improvisation, the recent new translation by Sidney Kravitz retains all of Verne's original detail on how the island dwellers use science to survive and thrive. I found it much more readable, and with almost a third more material than the shortened version I'd originally read - it seemed like a whole new book!

(If you're already a Verne fan, read some of the customer comments on Amazon for the Kravitz translation to get a sense of the changes.)