great american novel
well, if it were american it would be my candidate. the writing is as pure and elegant as i've ever encountered, yet full of dialect and idiom written with a perfect ear for speech and great humor. i recently re-read chapter two of the book and was again left mouth agape at its perfection. the plot is many-layered and tight, giving the reader the benefit of the perpectives of many characters, each with his/her own voice. if you've seen the "movie" and liked it at all, and if you enjoy reading, then the novel is an absolute must read.
as for the movie, i believe it is, as john-verity says, a faithful adaptation of the book, and the acting is superb. in the detective phase of the story, you will encounter many great british character actors whom you will recognize if you are a fan of the various wonderful bbc productions out there. if you like this sort of thing, i highly recommend "smiley's people" and "tinker, tailor, soldier, spy" in both the book and film versions.