A Tale of a Tub is certainly Swift's least classifiable work. He's best known, of course, for Gulliver's Travels. This work was mostly written at the very start of his career, when he hadn't yet totally hardened into his later misanthropy, and it has all the demented exuberance of a great writer in his mid-20s finding a voice. It defies description. The kernel of it is a satire on religious controversies, but that makes up about a third of the actual text. The rest is a series of prologues, forewords, dedications, prefaces, afterwords, epilogues and appendices, the sheer profusion of which suggest very much that Swift is poking dire fun at the idea of writing itself. In that respect, it goes further than any 20th century French golden boy of artistic revolt; Artaud looks like a stamped-in-tin romantic poet when set against Swift's manic nihilism. A Tale of a Tub is the closest anyone has ever got to writing a book that tackles head-on the futility of writing books, but that's only one interpretation of it. It exhausts interpretation by being as near as possible about nothing at all - and hence about everything. Plus it's not even 200 pages long. Swift never wrote as irresponsibly ever again, although the Travels, 'A Modest Proposal', the Bickerstaffe Papers, the 'Verses on the Death of Dr. Swift' and the Drapier's Letters are all admirable enough. A Tale of a Tub is as comprehensive a piece of literary terrorism as was ever attempted.
Swift, the greatest English satirist, is of course best known for Gulliver's Travels, but the Tale of a Tub is more complex, more vicious, and funnier. In some of the best prose of the 18th century, he ridicules all sorts of conventions, religious, literary, rhetorical, and otherwise. He makes full use of the capacity that prose has for being deliriously irrelevant and digressive. It is similar in some ways to Tristram Shandy and the novels of postmodernism. It'll give you fits.