Every book ever written by Neal Stephenson (Author), the majority of which were not "must reads" when I read them, but seem to have become so since, given the pace of technological and economic development the world over.
Seriously, people often ask me why I liked one of his books, and when I inquire how far they got, it's usually "a couple chapters" or "50-60 pages". I've found that for the vast majority of his works, one must read around 150-200 pages to become "hooked" enough to barrel through the rest (often at great detriment to sleep and dietary habits). But it does seem to happen quite reliably every time.
Which "must read" books are difficult to finish or "get into"?