Chaos Reading discussion

Only Revolutions by Mark Z. Danielewski came up in another GR group and it got me thinking about what books I've struggled with (his is not one; well, not yet--I've opened it a couple of times and read a 3 or 4 pages, but then put it back just not ready to try and tackle it). But there have been books that have taken me 3 or 4 attempts to get through (Das Kapital and Anti-Oedipus: Capitalism and Schizophrenia), or books I've forced myself to finish for one reason or another.

What books have you struggled with and why did you put yourself through the ordeal? Was it worth it, in hindsight?