what are 3 key words used in the text?

Books - obviously the people have no knowledge of books and have no desire to have books.  The fact that they are set on fire shows us a world in which life is very different.

Fireman - A fireman in the world of the reader is one who puts out fires.  There is no need for such a person in the world of the novel since he is one who sets fires, mostly in the houses where books are hidden and where secrecy is the lifestyle

Suicide - Mildred's attempt to take her life mirrors a more serious problem in the lives of the people; the men who ultimately save her do such work routinely, thus indicating that people see little reason for living and thus are more content to think about dying.