"natural monopoly" is economist's jargon. It is basic jargon. Every undergraduate economics text book covers it. Mankiw's Principles of Economics discusses it on page 306. Economics by Parkin and King cover it on page 296. Beardshaw sees to have something against the term, talking instead about "The flat bottomed average cost curve" on page 266. Alchian and Allen discuss natural monopoly on page 290 of Exchange and Production. Google it.