“Themistocles was regarded everywhere as by far the wisest man of all the Greeks; and the whole country rang with his fame. … Themistocles was given a crown of olive too, as the prize of wisdom and dexterity. He was likewise presented with the most beautiful chariot that could be found in Sparta.”
~ ‘The History’ (Book 8) by Herodotus (George Rawlinson transl. GB6 - p. 282)
The book ending soon after the Greeks declare, “So long as the sun keeps his present course, we will never join alliance with Xerxes”, as a response to the looming threat of another Persian invasion, was a perfect cliffhanger that makes a person want to continue to the next book. To Alexander of Macedon (the great-great-great-great-grandfather of Alexander the Great), who comes to the Athenians with the offer of alliance, this doesn’t make rational sense. Everyone knew that the Persians had already burnt Athens to the ground in the last invasion and could wreak havoc again. Alexander likely felt he was acting in good faith, trying to save a stubborn people from total annihilation. At the end of the book, one is left thinking - what will happen? Will Mardonius come charging with his large army and crush the Greeks? Will the Greek alliance be able to hold off the attack?
(I do agree that Herodotus had the whole volume of histories as one book, and later it was split into nine books. But each book was kind of self-contained, I thought. The first ended with Cyrus the Great’s rise and death, the second about the Egyptians, the third about Cambyses’ awful reign and the rise of Darius, the fourth about Darius’s conquests and so on. Book five sets up the Ionian revolt, and one does get a foreboding that things are going to take a bad turn after that.)