Jesus and the Jewish Roots of the Eucharist, by Brant Pitre is one I would recommend.
Catholicism didn’t just suddenly appear in time and history. The book will put the Eucharist (and the Mass) in context of salvation history, as givn by the Old Testament.
And he is understandable.
Someone suggested the Catechism of the Catholic Church. I would not suggest starting there as you will soon get in over your head. However, Michael Pennock, in This is Our Faith provides commentary on the paragraphs of the Catechism in an easy to understand format. The subtitle is A Catholic Catechism For Adults.