It depends on the theme. If we're picking something in a space the group already knows well, like databases, I'll look at "Best Papers" from recent VLDB/ICDE/SIGMOD conferences. If we're exploring a topic most people are unfamiliar with, we'll go with something more foundational instead. For example, we're starting an arc on datacenters (servers, racks, networking, load balancing, power, cooling, failures, etc.), and most attendees don't have deep background there, so I found a book on the topic that we're going to read through[1].
[1] https://link.springer.com/book/10.1007/978-3-031-01761-2