The Cloud of Unknowing is my favorite monkly advice, though possibly because it's been given a very nice modern English translation (other stuff might be just as good if treated with as much interpretive freedom?). While you'll find immediately that it's talking about attention during prayer, the way distraction is discussed is still really generally useful. Some excerpts here: https://www.theculturium.com/the-cloud-of-unknowing/#attachm...
Thomas Aquinas talked about attention/boredom, I think, but I can't read him unless I'm in a very particular mindset so I don't have much to cite you.
Hm. I looked a bit at some different stuff but the problem is that I can't really recommend e.g. [The Imitation of Christ](https://en.wikisource.org/wiki/Of_the_Imitation_of_Christ) unless you're really looking to spend a while with medieval Christianity for its own sake, with the treatment of attention sort of incidental. I will think more and come back to this comment thread if I come up with a better rec. (I'll also note that a tragic portion of this stuff is dead-tree-only)
A priest friend recommended Thomas a Kempis to me, and I enjoyed listening to this audiobook version. Yes, it's not for the agnostics or the faint of heart, but it is an amazing explication of what it means to be a faithful layman and follower of Jesus.
