I have to disagree. For what I saw “efficiency gain” expression is used, and not “Cosine Efficiency Gain” as such. However the used words are in 18.2.4 subsection which is a part of 18.2 section whose title is Cosine Efficiency and its Maximization as I precised in my previous message. So one hardly can doubt that “efficiency gain” refers to “Cosine Efficiency”. So the concept of cosine gain is used in the book.
Another quote on the same page 449:
" However, as visualized in Fig. 18.7, a considerable efficiency gain is only achievable if the tower height to operation altitude ratio is not too small. Consequently, the efficiency gain and effectiveness of
a tower would be rather low for kite operation altitudes above hk ≈ 1000m."