A lot more expensive and a lot more comprehensive than Hoeppner´s book are
G. P. Neumann (Herausgeber): Die deutschen Luftstreitkräfte im Weltkriege (published 1920) and
Walter v. Eberhardt (Hrsg.): Unsere Luftstreitkräfte 1914-1918 (published 1930)
Very helpful is John H. Morrow, Jr.: The Great War in the Air
, Military Aviation from 1909 to 1921, published 1993. Gives a very good overview about the main powers Germany, Great Britain and France but reports also about the "lesser powers". As well the work is not limited to simplistic battle field review. It dicusses also the industrial, logistical and development problems.