I am looking for a comprehensible material covering Plancherel formula for $SL(n,\mathbb{R})$ and $SL(n,\mathbb{C})$. Of course, I wouldn't mind reading an explanation for general semisimple Lie groups. 


(I am reading Varadarajan's Introduction to harmonic analysis on semisimple Lie groups and I'm a bit lost there.)



Plancherel formula for special linear group
Knapp's book *Representation theory of semisimple groups : an overview based on examples* has a chapter on various versions of the Plancherel theorem. It is also done (very carefully, with all the details, I suppose) in volume 2 of Wallach's *Real Reductive Groups*. 


