I was all set to write up a piece on Garrett Jones, but found that Matthew Carruth at Fangraphs beat me to the punch. Carruth sets up the historical background and performance for Jones very well.
Looking through my past books, his first mention from me was in the 2005 Baseball Prospect Book, where he garnered a Grade C. I wrote that the improvement he showed that year in Double-A "could be for real" but that he was trapped behind Justin Morneau. I didn't put him in the 2006 or 2007 books. He re-appeared in the 2008 book, with another Grade C and a rating as potential "Brian Daubach" type player.
Jones' numbers this year look like a classic Age 27/28 explosion to me. As Carruth points out, his performance has been exceptional but well out of context for the rest of his career. This reminds me of what Shane Spencer did in 1998, hitting .373/.411/.910 with 10 homers in 67 at-bats for the Yankees. He never did anything like that again, though he had some use as a platoon player in subsequent seasons. Another, more optimistic possibility, would be a Luke Scott parallel: he hit .336/.426/.621 in 249 at-bats for the Astros as a 28 year old rookie in 2006, then has settled into being a solid productive slugger.
Obviously Jones will cool off....he can't maintain the current Super-Pujols pace, though he should remain productive the rest of the year and will probably finish with strong numbers overall. It will be interesting to see how the Pirates approach this, if they make him part of their long-term plans, or if they take a "wait and see" approach. I would advise them to expect a Spencer-like dropoff, and be pleasantly surprised if you get a Scott-like player instead.