Barnes presents what could very well be the only introductory to basic text that a civil or ground engineering student would need.
It presents all of the key, overarching concepts needed for soil mechanics, and generally in a more comprehensive, clear way compared to similar texts.
Good use of diagrams and good reference texts ensures that if Barnes doesn't have it, he knows who does.
The book also touches on, to some extent, the basics of foundation engineering, although deep foundations aren't dealt with too well.
Overall though I would agree with many academics that Barnes is the best introduction or basic reference for soil mechanics available.