The Screwtape Letters is a satirical work written by C.S. Lewis. It's a series of fictional letters from Undersecretary Screwtape to his nephew Wormwood -- a junior tempter -- whose task is securing the soul of "The Patient." People intent on proving their superiority and exercising Broca's Area for no apparent reason might call it an epistolary novel.
This has a number of spoilers, so first go out and read the book, then come back, gush profusely about how wonderful this is and 'cool' it.
It's full of helpful hints for securing the damnation of souls (as a result one pastor actually wrote to Lewis condemning the book as diabolical.) Lewis reportedly found writing the book very difficult, because Screwtape's vile character and constant stream of twisted negativity and vitriolic opposition to goodness was very difficult to maintain. Screwtape makes Mephistopheles look like an amateur. Screwtape is a professional (if sociopathic) salesman, rather than some laughable caricature of evil with horns; he's more of a bureaucrat than a Beelzebub.
Screwtape's view of what is "good" or "pleasant" is very different from what a humans might value; he calls God "the Enemy," he detests music and feels that it is "an affront to dignity and austerity of hell" because God is "really a hedonist at heart." His views are not directly opposite. He considers death bad and painful (as humans generally do) when it robs him of a good meal; at the end of the book 'the patient' dies and Screwtape is completely furious because the patient's death means that the patient is no longer available to "devour." Screwtape still doesn't understand what the plan behind claiming to love humans and die for them is, he senses an ulterior motive.
Depending on the time and place, who he was talking to and what effect it would have on his job, Screwtape would probably recommend aspiring fiends to use the Letters merely as a supplement to the admirably twisted curriculum at the Young Tempter's College. While not exactly a how-to manual (a Tempting for Idiots book would probably have amused Screwtape) the bulk of the book is concerned with the finesse required for keeping the Patient away from God, at first by trying to prevent his conversion and then attempting to nullify it by making the patient into a hypocrite. This is not merely Screwtape's personal theory (heterodoxy is encouraged only when it opposes the Enemy) many people can testify to the persuasiveness of pharisaical lectures of any kind, whether religious or not. His recommended tactics are many and varied, including confusing the patient, cultivating the patient's indifference, using coordinated attacks to make the patient's already strained relationship with his mother worse and appealing to the patient's sense of superiority.
Lewis tried to get across what he had learned through personal experience and observation of human nature. Screwtape's sarcastic characterization of humans as amphibians (partly spiritual and partly animal) is probably related to Lewis's views on the soul, though obviously Screwtape's goal is to maim, torture and otherwise mangle a person and he studies them with only that end in mind.
It was originally published as a serial in the Church of England magazine, The Guardian between May and November 1941.
John Cleese narrated a version of the Screwtape Letters. There's an audio drama where Gollum plays Screwtape -- which is not terribly relevant, I just added that because I like Andy Serkis.
The Screwtape Letters was also adapted into a play.