Thanks, that's clear
The airstream is turned downwards for various reasons after passing over the wing. The tail sits in the downwards-turned airstream, which is for the tail its "free" airstream. DPD uses "epsilon" for the angle of downwards-turning, aka downwash. Let alpha_0 be the angle between the wing chord and tail chord. Then the incidence (=angle of attack) of the tail is (alpha - epsilon) + alpha_0. This is all on p120. This is all pre-stall terminology.