The Crying of Lot 49 by Thomas Pynchon is often regarded as an example of literary postmodernism. Just as modernism undermined many of the older certainties of positivism, so postmodernism attempts to deconstruct the underlying assumptions of modernism.
In your essay, you should begin by discussing the particular themes and theories of modernism that are the focus of Pynchon's deconstructive project. Perhaps the most appropriate for this novel would be psychology.
Modernism undermined many of the certainties of previous thought by accepting the insight of Sigmund Freud that many of our motivations are not conscious or rational, but rather the product of the secret forces of our subconscious minds, often in response to the traumas of childhood. Pynchon, on the other hand, performs a postmodernist interrogation of psychology itself.
The protagonist's name, Oedipa, suggests Oedipus, but inverts the Oedipal notion by being a female Oedipus (not an Electra, which would have been the Freudian term). Her own psychotherapist, rather than being a normative and normalizing figure, is himself insane. Paranoia, rather than being a disease of the mind distorting Oedipa's worldview, is a precondition of understanding.
Rather than accepting a binary of sane/insane, the novel suggests that paranoia is the act of seeing connections. The improbable connections in the plot -- including the multiple readings of acronyms -- are not resolved, but are infinitely deferred. It is in the reader's making of connections, between, for example, medieval Taxi services and modern finance, that the plot is constructed, but at the same time, plot may simply be evidence of plotting, either on the part of worldwide conspiracies or of the reader's paranoia.