In order to define an author's particular style, we must, in part examine the degree by which the author leans towards different ways of putting things (technical vs non-technical vocabulary, long sentences vs short sentences and so on). This task requires comparisons to be made not only internally within the author's own work, but also with other authors or the norms of the language or variety as a whole. As Leech and Short (1981) point out, stylistics often demands the use of quantification to back up judgements which may appear subjective rather than objective. This is where corpora can play a useful role.
Another type of stylistic variation is the more general variation between genres and channels - for example, one of the most common uses of corpora has been in looking at the differences between spoken and written language. Altenberg (1984) examined the differences in the ordering of cause-result constructions while Tottie (1991) looked at the differences in negation strategies. Other work has looked at variations between genres, using subsamples of corpora as a database. For example, Wilson (1992) used sections from the LOB and Kolhpur corpora, the Augustan Prose Sample and a sample of modern English conversation to examine the usage of since and found that causal since had evolved from being the main causal connective in late seventeenth century writing to being characteristic of formal learned writing in the twentieth century.
Read about stylistic work carried out by Biber and Wikberg in Corpus Linguistics, Chapter 4, pages 102-103.