If watching the movie is your first introduction to Firefly, as it was for me, then you don't really get much opportunity to know Book before he dies... for a first timer there's not much difference between Book and Mr. Universe, and their deaths elicit about the same emotional response.
Wash, on the other hand is presented as a member of the core-team and the audience gets to know him well in the movie so when his death happens it has much more impact. And, as mentioned, there is the impact on Zoe and the suddenness with which he dies. In fact, there's a dramatic pause which makes the moment more poignant: he succesfully crashlands and it looks like he pulled off the impossible but in that split second later he gets hit and you think "what? what just happened?" and in the next split second you realize that he's just received a mortal wound from the reaver spear/whatever it is.
I think it was brilliant to kill him off... his death heightens the uncertainty (for the audience) of the other characters' survival during the reaver attack.
Even after having all the episodes of Firefly, and getting to know Book much better, Wash's death still has more impact than that of Book when I watch Serenity. Perhaps its because he gets alot of the comic relief lines and his relationship with Zoe...
That said, Book gets some pretty good lines too "a special kind of hell", and the whole scene where River is terrified by his loose hair. It's clear that Whedon had a back-story on Book but due to cancellation never got a chance to flesh it out.
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