I read this casually, a little at a time. It's one of the great adventure stories of all time, and smashing stuff (get it?) but...here's how it works: it's based on the journals of Shackleton and everyone else in his party - he gives others lots of time too - and the entries can be a little repetitious. Like, y'know, "Still stuck on an iceberg. Cold and hungry."
Shackleton's a surprisingly good writer, though. Clear, engaging and often funny. That livens up the doldrum periods - but also, the effect of the long passages in which nothing dramatic happens is that when something does happen, it happens with extraordinary, direct impact. His account of - minor spoiler, I guess? - the final destruction of the Endurance is just crushing. An incredibly powerful moment. The immediacy of the epistolaryish format, with its you-are-here feel, makes the big moments of the expedition directly heartbreaking.
After his account of the main expedition, he starts completely over with what happened with the other boat, the Aurora. (You will have forgotten they exist by this time.) This is a tough one; it's just as compelling a story - they actually had it worse,
if you can believe that, and again it's based on journals so it has that you're-right-there! feel to it, but there's no avoiding the fact that, having slogged all the way through Shackleton's brutal story, you groan a little when you realize you're about to start over.
I guess I'd suggest laying it aside and picking it up later for this part. It is much shorter, at least. And it's much shorter even than it looks, because after the story of the Aurora's landing party (again, this really is great stuff on its own), Shackleton backtracks again,
to the people who stayed on the Aurora, and that part is utterly skippable. Nothing whatsoever happens. I read it so you don't have to. Just stop there.