Highly recommended! Fr. Adrian Nichols, OP: Conciliar Octet: A Concise Commentary on the Eight Key Texts of the Second Vatican Council

"In Sacrosanctum Concilium the theology of the Liturgy is chiefly indebted to Pius XII's encyclical letter of 1947 on the same subject, Mediator Dei. No one coming unadvised to the conciliar constitution would guess this, owing to the surprising failure to cite the letter among the references given in in the constitution's printed form. The footnotes refer exclusively to the sources of Mediator Dei rather than to Mediator Dei itself". 

It is a commonplace among the liturgically discontent that the rot set in with Pius XII. But  Fr. Nichols' point is different: there was intentional continuity between Mediator Dei and the Constitution on the Liturgy such that one  should be understood in the light of the other. 

In any case, read it!