Books about coins are almost as interesting as the coins themselves. I will only list here the references I own myself. They're not necessarily the best, but the ones I have to hand. I have about 150 references, so it will take a while to get them all listed with some info. on them.

Categories

Roman Republican Coins

Sydenham - Coinage of the Roman Republic

Crawford - Roman Republican Coinage - the best reference today

Sear - History and Coinage of the Roman Imperators

Seaby/Sear - Roman Silver Coinage (vol. 1 of 5)

Sear - Roman Coins and Their Values (vol. 1 of 5)

Roman Imperial Coins

Roman Imperial Coinage (RIC) - 1926-date (10 volumes)

British Museum Catalogue of Coins of the Roman Empire (BMC) (6 volumes)

Sear - Roman Coins and Their Values (5 volumes - Millennium edition, older single-volume edition available)

Seaby/Sear - Roman Silver Coinage (5 volumes)

Greek Coins

Sear - Greek Coins and Their Values (2 volumes)

Hoover - Handbook of Greek Coinage (work in progress - will eventually be 14 volumes)

Head - Historia Nummorum