Challenging and full of ideas; I'm still chewing on it, two weeks out. If you haven't read dense fiction in awhile, I'd start with the stories in Part Two, then track back to Part One. Wasn't as much a fan of the ones that were meta-literary criticism, though these have many clever bits. All are stories within stories, intricate and clever. Faves include The Secret Miracle (about a writer!), The Garden of Forking Paths, The South (which Borges says in his intro is his best; I might disagree), Theme of the Traitor and Hero (for writers), Three Versions of Judas. I'd read Pierre Menard for a class, which ruined it for me; I didn't think I liked Funes, the Memorious but thinking back on it now I like it more.
“In all fiction, when a man is faced with alternatives he chooses one at the expense of the others. In the almost unfathomable [fiction of] Ts’ui Pên, he chooses— simultaneously—all of them. He thus *creates* various futures, various times which start others that will in their turn branch out and bifurcate in other times. This is the cause of the contradictions in the novel.
"Fang, let us say, has a secret. A stranger knocks at his door. Fang makes up his mind to kill him. Naturally there are various possible outcomes: Fang can kill the intruder, the intruder can kill Fang, both can be saved, both can die and so on and so on. In Ts’ui Pên's work, all the possible outcomes occur, each being the point of departure for other bifurcations. Sometimes, the pathways of this labyrinth converge. For example, you come to this house; but in other possible pasts you are my enemy; in others, my friend.” -- The Garden of Forking Paths, p 98