Catechisms on the Sacrament of the Lord’s Supper
John Willison’s “A Sacramental Catechism” is a household favorite, we use it often around communion seasons. You can download the book for free from Google Books, or buy an inexpensive copy from RHB.
Also, check out James Fisher’s “The Assembly’s Shorter Catechism Explained“ regarding the Lord’s Supper:
Q96: What is the Lord’s supper?
Q97: What is required to be the worthy receiving of the Lord’s supper?