About the circulation of books, in his "Lives of Eminent Philosophers" Diogenes Laertius tells this anecdote about Socrates:
They relate that Euripides gave him the treatise of Heraclitus and
asked his opinion upon it, and that his reply was, "The part I
understand is excellent, and so too is, I dare say, the part I do not
understand; but it needs a Delian diver to get to the bottom of it."
Book II, Chapter 5, 22
Besides that Heraclitus can get really deep, we can infer from this that book lending in Classical Athens was not unheard of.
The source for this anecdote seems to be Aristo of Ceos, a 3rd-century BC philosopher, as Diogenes Laertius explains in the Herodotus chapter:
The story told by Ariston of Socrates, and his remarks when he came
upon the book of Heraclitus, which Euripides brought him, I have
mentioned in my Life of Socrates. However, Seleucus the
grammarian says that a certain Croton relates in his book called The
Diver that the said work of Heraclitus was first brought into Greece
by one Crates, who further said it required a Delian diver not to be
drowned in it.
Book IX, Chapter 1, 10