lugubrious
Mournful or sad; often associated with melancholia induced by too much alcohol.
reason
Being rational; the novel is at pains to point out that much of what we do and consider reasonable is in fact merely habitual.
release
A sense of liberation, as in "I think it may have come from a sense of facing something quite new and fresh to me" (p. 60)
Triffid
The genetically engineered plant, quite probably of Russian origin, which gives the book its name, and its villain.