Forecasting: Principles and Practice

There are a lot of practical implementation scenarios for Time Series forecasting such as sales forecasting, Demand forecasting, energy forecast etc. However, forecasting has a lot of mathematical background such as Statistical Modeling, Smoothing etc. Opensource book by  Rob J Hyndman and George Athanasopoulos at Monash University, Australia titled Forecasting: Principles and Practice is trying to fill the conceptual gap on the topic. 

Read the open source book at https://otexts.com/fpp3/index.html. The book covers, ARIMA Models, Regression models, Smoothing Techniques, and some practical examples using R.