The second volume of Edward St. Aubyn’s Patrick Melrose series contains some of the best and most realistic representations of drug use I have found in literature. The aristocratic Patrick Melrose, whose ancestors were on the winning side of the Battle of Hastings, flies to New York on the Concorde, sets himself up at the Pierre Hotel, and takes a cab down to Alphabet City to top off his heroin supply. It goes downhill from there.
If you are looking for an alternate view of the British aristocracy or if you consider the privileged class a haven for buggery and hard drugs, then Bad News is for you (but read the first volume, Never Mind, first).
For more on this compelling author and the complete Patrick Melrose series, see Edward St. Aubyn.