The best option in English is the FSI Greek Basic Course, which was created by the US government when katharevousa was still the official language of Greece. Volume 1 teaches standard modern Greek and points out some of the most common elements of katharevousa that one is likely to encounter on the street and in conversation. Volume 2 gradually transitions to katharevousa and teaches grammatical elements. Volume 3 is essentially a reader of katharevousa as it was used in the newspapers of the ‘60s. That would give a solid foundation to start reading katharevousa, beyond that you would need to study some Ancient Greek to read literary works.