https://www.otexts.org/fpp is my reference. It's free and good.

Forecasting: principles and practice by Rob Hyndman and George Athana­sopou­los [1] is my reference. It's free and good.

[1] Hyndman, R.J. and Athanasopoulos, G. (2013)
Forecasting: principles and practice.
OTexts: Melbourne, Australia.
http://otexts.org/fpp/. Accessed on 11 April 2017.