Whether due to its jargon inspired hermetic highly latinized vocabulary or the fragmentary, idiosyncratic, elliptic style partaking to the stream of consciousness or the esoteric nature with the additional impenetrable and indissoluble themes, some book's difficulties are more conspicuous than others', and the sheer amount of effort they require in order to be accordingly perceived, which are some of the most difficult works you've read, or tried to read for that matter? Some options are obvious, Ulysses, Finnegans Wake, The Sound and The Fury and other modernist projects. There are, of course, some other that, while yet quite traditional, are bordering on modernism, like James' The Ambassadors, and are still difficult to tackle.