Unbeknownst to many, Ancient Greek, spoken roughly from 9th century BCE to the 6th century CE, can be subdivided into several periods, i.e. the Archaic period, the Classical period and the Hellenistic period (followed by medieval Greek). Plato's dialogues were written the Classical period. For this reason, it would be useful to find resources for the autonomous study of Ancient Greek from the Classical period. I think quite a few textbooks are available, but I don't know how many of them are suitable for self-study. (These resources needn't be in English; German and French are also fine.)
Here are some textbooks found online for self-study of Classical Greek. All of them are English books which have exercises included. Most of them are only in Attic, but I marked those which I could tell included other dialects.
Based on Xenophon:
Based on readings (from classical sources or original):
- Athenaze (available for borrowing, includes non-Attic Greek)
- First Greek Course, First Greek Reading Book, Greek Prose Composition (three parts)
- First Greek Course, A Greek Reader, A Greek Boy at Home, Vocabulary to A Greek Boy at Home (the third book corresponds closely to the lessons in the first book, the second doesn't seem to)
- First Greek Book (readings from classical authors, including non-Attic dialects)