A number of translations of the current version of the Institutio Generalis Missalis Romani – but not the Latin text – is available on the Vatican website. (I have no idea why they give the ungrammatical title "Institutio Generalis Missale Romanum" which means "general instruction the Roman Missal" perhaps somebody was spooked by the fact that the genitive of Missale – Missalis – has an identical ending to the nominative Generalis). This is known in English speaking circles by the acronym GIRM for General Instruction of the Roman Missal The almost twenty year old website Christus Rex provides the predecessor of the current GIRM, promulgated in 1975. Helpful, for reasons hinted in the title to this post.