Meetings:
Further reading (apart from the Wikipedia entry):
Jean-Pierre Serre: A course in Arithmetic
Neil Koblitz: p-adic numbers, p-adic analysis, and zeta-functions
Fernando Gouvea: p-adic numbers: an introduction
Andrew Baker: Online course notes
David Madore: Online notes (6pp, some explicit examples)
Wikipedia: Statement
Wikipedia: Proofs
G. Rousseau: On the Quadratic Reciprocity Law
Mathpages: Short statement with proof
Further reading
Don B. Zagier Newman's short proof of the prime number theorem