A very nice exposition is given in I.G. Macdonalds [Spherical functions on a group of p-adic type](http://www.amazon.com/Spherical-functions-group-p-adic-type/dp/B007EQFY8Q/ref=sr_1_1?ie=UTF8&qid=1453731686&sr=8-1&keywords=spherical+functions+on+the+groups+of+macdonald)


Another nice exposition is given in the section 3 of the [these notes](http://math.univ-lyon1.fr/~thuillier/articles/BerkoParis.pdf).

Garret's book is perfect for learning about buildings in general, while these notes summarize exactly how they one constructs a building for a reductive group.