In Fahrenheit 451, how is Clarisse a presence in the book after her death?
In Bradbury's Fahrenheit 451, one element of the work that keeps Clarisse in focus long after she appears for the final time is the omitting of any details concerning her fate. Unless I'm missing something (which is highly possible), Clarisse doesn't die. She disappears, but no details are given. The reader doesn't know what happens to her, so the reader keeps wondering.
Montag, presumably, wonders too. He mentions her often and continues to be influenced by her throughout the work. Montag never finds out what happens to her, and neither do readers.
A tragic and sensational death can be remembered throughout a work, too, as the woman who burns herself along with her books demonstrates. But that death serves more as a momentary catalyst for Montag. Clarisse, whose fate is unknown, remains a presence throughout the rest of the novel. A good reader wants to know her fate, and is, presumably, looking for it for the rest of the novel. Her fate is never revealed, which keeps the reader wondering.
In my opinion, Clarisse is a presence after her death because she continues to be an inspiration for Montag.
You can see Montag continuing to think about Clarisse even after the last time he sees her. He sort of quotes her to his fellow firemen (when he says that they used to fight fires). He thinks about her and talks to his wife about her. He thinks about her when talking to Beatty, when he talks about front porches and people just wanting to talk.
All in all, what is happening is that Clarisse's words and actions have really stuck with Montag. He has taken her thoughts to heart and they influence his actions over the rest of the book.