What was the Song in the book?

The Song, better known as the Song Tra Bong, was a river.

At one end was a small dirt helipad; at the other end, in a rough semicircle, the mess hall and medical hootches overlooked a river called the Song Tra Bong. Surrounding the place were tangled rolls of concertina wire, with bunkers and reinforced firing positions at staggered intervals, and base security was provided by a mixed unit of RFs, PFs, and ARVN infantry. Which is to say virtually no security at all.