The only extant examples of written Brythonic that we're currently aware of are two curse tablets found in the sacred pool of the Aquae Sulis temple/bathhouse complex at the centre of modern Bath. Actually identifying them as Brythonic was the result of significant academic debate as the tablets are written in the Latin alphabet; as a result, we don't have any evidence as to whether there was an original Brythonic alphabet. While curse tablets are a fascinating class of artefact, they typically only take the form of a name - or list of names - and often a brief summation of the infraction that the petitioner wishes to bring to that shrine's deity's attention. As a result, while curse tablets are evidence of written Brythonic, they sadly don't provide us with any knowledge of pre-Roman literature.

You can read an engrossing article about them here