Once upon a time I finished every book I started. Can't remember now if there was any particular reason for this other than I had a lot of time and only chose books that seemed interesting in some way. Which seems kinda obvious but one benefit of true nerd-dom is a lack of peer pressure to keep up with whatever you "should" be doing. (And of course is why I hit college with a phenomenal amount of SF and fantasy under my belt but not much in the way of classics or mainstream reading, a situation that's pretty much reversed in the couple of decades since then.) Though I'm sure somewhere there must have been books long forgotten that I never finished the first I remember deliberately giving up was Huxley's Point Counterpoint which was assigned for a class when I was a college sophmore or junior. Though I think it's something I might like now, at the time it seemed turgid and pointless so I dug up some reviews and critical pieces, synthesized that into my own paper and got a "B" for it.
The reason this comes up is that I just gave up on Carlo Emilio Gadda's That Awful Mess on Via Merulana after making it about 80 pages (of nearly 400). This sounded like My Kind of Book: a philosophical mystery compared to Joyce with lots of local color and obscure references. I'm a sucker for mysteries with that high-brow art attitude: Eco, Auster, early Peter Dickinson, even Chesterton. And that description of Awful Mess is more or less true except that I didn't get to much philosophy. One problem is that all the characters blurred together even though I stopped and actually started over which sorted some of them out but now they seemed like pretty much just names (perhaps more proto-Barthesean than it should have been). And though the multiplicity-of-voices approach does seem promising in practice it was just too much work reading for very little payoff. So I didn't stop reading because it's "bad"--for all I know another 30 pages and it would all click into place--but because I realized it wasn't entertaining/interesting ("entertaining" being the lowbrow and "interesting" the middlebrow words for the same thing) enough to give up that time when something else could be in its place, and because I'm not gaining anything by finishing it.
This past summer I similarly gave up on W.J. Cash's The Mind of the South for somewhat the same reasons. This is a book I'd wanted to read for years and finally in one of those inexplicable moments took it out of the library and started. This one was even more clear cut because as pure writing it was neither here nor there so it came down to what did I hope to gain by finishing it? All Cash's talk of Cavaliers and hill folk was removed from any kind of reality I've seen in 40+ years as a Southerner or could even match in any meaningful way to even more imaginative discourses so in the end it seemed like all I would learn is a piece in the story of how the South has been perceived and even created as a conceptual category. Which, really, I don't much care about.