I read Design Patterns. The reason it was difficult for me was because they introduced many entities at once, and I couldn't keep track. My solution was to draw a diagram of the entities and their relationships as they were mentioned, and take some time to try to absorb them. While not a mathematical paper, it's closer to it than to regular prose, in that it needs to be studied rather than read.

(BTW: I didn't find it all that interesting conceptually, they're the solutions you'll come up with yourself, just collected and named and categorized. Reading how they solved specific problems was interesting - vicarious experience - though some seemed more whimsical than workmanlike.)