This may seem obvious – but I’d recommend Sacred Scripture in conjunction with a missal, to get the context of all the Scriptural references that make up the Mass. A person familiar with the Bible but not so much with the Mass may be amazed to find how much Scripture is packed into the Mass – not just in the readings, but in every prayer, every posture, and every exchange between the priest and the people.
I’d also recommend a good guide to the Mass, such as those by Fulton Sheen, Peter Kreeft, etc. When you’re aware of the background behind each liturgical word and action, it greatly enhances the experience of worshipping God in spirit and in truth.