It seems there is some error in the momentum expansion in “Expanding the velocity variable v_t” (eq. 12.6.2)

Fixed. Stay tuned for a complete rewrite of the chapter (it should go live in a few hours).

In the line below eq 11.6.7, ∂xf(x) shoud be ∂xh(x)? And (x − Q-1c) in eq 11.6.7 and following lines, should be (x + Q-1c)?