The basic sources for Catherine’s life and ministry remain the biographies written soon after her death by her followers, including Raymond of Capua’s Life and Thomas of Siena’s Supplement.
There is an adequate English translation of Raymond’s life: The Life of St. Catherine of Siena (tr. George Lamb) TAN Books, 2003.
Catherine’s own writings have been collected in critical editions and translated. The Dialogue, in particular, was first translated in English in 1519; the best modern translation is probably: Catherine of Siena, The Dialogue (tr. & ed. Suzanne Noffke, (Paulist Press, 1980). Her letters can be found in The letters of Catherine of Siena, 4 vols, (tr. Suzanne Noffke) Arizona Center for Medieval and Renaissance Studies, 2000-2008. There are also collections of her prayers available.
A good starting point for studying Catherine seriously is: Thomas McDermott, Catherine of Siena: spiritual development in her life and teaching Paulist Press, 2008; see also Carolyn Muessig, et al. (eds), A Companion to Catherine of Siena, Brill, 2012.
Her Dialogue can be found online in an older, and somewhat inadequate, translation here.
Photographic reproductions of an illuminated manuscript of her life can be found here.