Hampson, G.F. (1894 & 1895): Fauna of British India. Moths Vols II & III: 161–582; 1–104. You could download the volume in British Heritage Library. Available in online.
There is no one key to all the world species of Agrotis. Lafontaine & Poole (2004) give a key for all the North American species - the publication is available at: https://images.peabody.yale.edu/mona/27-1-ocr.pdf and the key is on page 225. German San Blas (2014) reviews all the south American species, with keys, at https://www.biotaxa.org/Zootaxa/article/view/zootaxa.3771.1.1 The Hampson Fauna of British India volumes are available from the BIODIVERSITY Heritage Labrary (run by the Smithsonian) using a simply Google search, but they are very outdated and cover only Indian species. You appear to be in Brazil...