I just finished reading the Penguin edition after midnight last night, and got so ...intrigued by the account of the Oligarchic revolution that I just *had* to post.
Rather than type it all in, I decided to go with the Gutenberg edition.
Speaking of that, it crossed my mind that the parsing into Chapters must be somewhat similar to other ancient writings and New Testament stuff, conventions are formed for divisions of the text, which choices harden over time.
Cheers!
Yes, it is very much like with editions of the Bible--standard divisions of the text make it possible to find a passage without having to use the same exact edition. And like with the Bible, the English version isn't the original version--and there are translations in many other languages. A French or German translation of Thucydides will use the same divisions. The usual English translations just have book and chapter divisions but some editions will further divide the chapters into sections (also pretty standard, but sometimes the Loeb edition will be a little different from the Greek-language editions published in Germany or France).
With Plato there are standard page numbers and sections of pages which go back to an early printed edition by H. Stephanus.