I like Earthdawn and own the second-edition rulebooks from Living Room Games. There seems to be a third edition. Is it worth it to buy these books too? To answer this question for me, I want to know more about the changes. What is new in the third edition, what has changed, what is missing? How does it influence gameplay?
Earthdawn has been published four times, by three different companies. It was originally published by FASA, and then the license somehow got into the hands of both Living Room Games and RedBrick Limited. LRG made their changes to the game and published the "2nd edition," which you'd be familiar with. RedBrick focused on reorganising and editing the original material, releasing their version as "Earthdawn Classic", then on expanding the line with setting supplements.
That was the state of affairs until recently—two companies with two similar but not-quite-compatible games both called "Earthdawn", and players of Earthdawn divided among the original game, the mostly-compatible Classic RedBrick edition, and the updated LRG 2nd edition.
Now, the third edition's place in that context might be enough to answer your question for you. The 3rd edition is produced by RedBrick (published through Mongoose), and is RedBrick's idea of how Classic should be improved to make for a better play experience. As such, the 3rd edition is unrelated to and doesn't build on the heritage of Living Room Games' 2nd edition, which is what you're playing. It's a separate evolutionary branch, so to speak. In terms of "steps of improvement" from the FASA edition, 2nd edition and 3rd edition are the same number of steps away from FASA Earthdawn, but in somewhat different directions. Which one to play is then a matter of personal preference.
The editions, in chronological order, are:
The changes in 3rd edition compared to 2nd edition haven't been itemised anywhere (that I know of) because they're two different companies' vision of what original FASA Earthdawn could become. However, there was an article by a RedBrick employee about the differences between Classic/original and 3rd. Annoyingly, the article is now lost in a site reorganisation, but some of the relevant points were preserved in a quote in this RPG Site thread on Earthdawn edition differences.