eh he modes
Better than "Microwave Horns and feeds. A.D.Olver etc. 1994" is the "Corrugted horns for microwave antennas" - P.J.B. Clarricoats and A.D. Olver - Peter Peregrinus ltd - ISBN 0 86341 003 0
Also on IEEE website look for Articles by James, G.L. between years 1970 and 1990.
Just to add to mudrik's answer, EH Hybrid modes are looked for because, inside circular waveguide and circular horn, E field lines are very parallel each one to the other one. This help to reduce X-pol and rise the system illumination efficiency.