The Frommer's guidebooks will have the appropriate information specific for your destination. Travelers to Europe will be happy to learn that in 12 countries, the Euro is now required currency, so you won't have to convert from, say, French francs to German marks when crossing the border between those two countries. The nations that have begun using the Euro are Austria, Belgium, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, the Netherlands, Portugal and Spain. The Euro at time of writing is worth about 90 U.S. cents, so for rough purposes, you can think of it as being about a dollar.
It's a good idea to exchange at least some money--just enough to cover airport incidentals and transportation to your hotel--before you leave home, so you can avoid the less-favorable rates you'll get at airport currency exchange desks. Check with you local American Express or Thomas Cook office or your bank. American Express cardholders can order foreign currency over the phone at tel. 800/807-6233.
It's best to exchange currency or traveler's checks at a bank, not a currency exchange, hotel, or shop.