I am unable to understand how is demand systems estimation is different from simple demand equation estimation. What's the best resource for getting a history line of demand systems?
One advantage of considering demand systems is that more data is available for identifying the degree of income effects and substition between commodities. A second advantage is the possibility to impose and test microeconomic restrictions (which are between equations). An introduction to the estimation of demand systems with an historical perspective is provided by:
Berndt, E. R., 1991, The Practice of Econometrics: classic and contemporary, Addison Wesley.
More recent treatments are available in most econometric textbooks.