Isabella; or, The Pot of Basil has a tighter sense of narrative control, a more distinctive and independent stylistic identity, and a defter interweaving of symbol and story than anything Keats had hitherto written. It is also darker in feeling, and at once more confidently literary and more distinctly odd. The mannerisms that afflict Endymion and test the reader's patience are in Isabella evidently posed and conscious. The awkward rhymes, arch medievalism, and eccentricities of diction, phrasing, and syntax appear as parts of a deliberated stylistic texture; in comparison with all of Keats's preceding work, perhaps excepting the Chapman sonnet, Isabella has a much more focused and purposive tone. But it is nonetheless strange for that. Its grotesque plotting, taken from a tale in Boccaccio's Decameron, is turned by Keats to unexpectedly suggestive and complex thematic purposes. The tensions and paradoxes latent in his writing of the past year now animate a narrative manner that combines abstraction from Keats's own everyday social world, with a most telling, if oblique, commentary on the underlying contours of that reality.

The poem's dark side is foreshadowed in the verse letter that Keats wrote to John Hamilton Reynolds on 24 March 1818, from Teignmouth. This poem is in the relaxed conversational manner developed in the Margate epistles and ‘Sleep and Poetry’. It begins as a light-hearted and fanciful effort to cheer Reynolds up on his sickbed. But after some sixty lines that recall ‘Sleep and Poetry’ in their evocation of escapist dream landscapes, prompted by imaginary scenes in literature and painting, the relaxed manner darkens and Keats acknowledges that his delight in sensuous and imaginative pleasures cannot be easily sustained in face of the manifest contradictions, evils, and vexing mysteries of experience. This acknowledgement brings with it a change of stylistic key, as the established manner becomes more weightily reflective. The couplets are sustained, but the verse takes on the meditative movement of a Shakespearian soliloquy (recalling Hamlet in particular), which pulls against the light feel of the rhymes: