That's a good one too, I enjoyed it also. One of the reasons I recommend Wert's book though is that Piston spends a whole lot more time on Longstreet's post-war travails, with just enough coverage of him during the war to set the stage. Wert goes much more in depth into Longstreet's Civil War career, with only a chapter or two (if I recall correctly) at the end on the post-war period. So both good books, but each with a bit different focus... so randoke maybe it depends on what you're looking for... or just get both!!
The other reason I liked Wert's book is simply that it was written after Piston's, and Wert had the benefit of some additional research and insights, etc.