D. P. Davies's "Handling the Big Jets" has a section on why higher is better for jet transport aircraft. Its a book that is well worth getting hold of, although it is now a bit old.
Mik
p.s. the reason I'm not going to quote Mr. Davies's explanation is that a) it's fairly long, and b) it includes graphs.
[This message has been edited by mik (edited 19 June 2000).]