Extra textbooks may be useful, especially because they contain extensive bibliography
Asymptotic Methods,
Henry van Roessel and John C. Bowman, University of
Alberta, (2012).
Advanced Mathematical Methods for Scientists and Engineers;
Asymptotic Methods and Perturbation Theory, by Carl M. Bender
and Steven A. Orszag, 1st edition, McGraw-Hill, 1978, 2nd ed.,
Springer 1999.
Applied Asymptotic Analysis, by Peter Miller, AMS, Grad. Studies
in Math, Vol 75.