Latvia is a member of the European Union, and as such most nationalities, including US, Canadian, Australian and Japanese, do not require a visa for visits up to 90 days. However, it is recommended that anyone wishing to visit Latvia should contact the Latvian consulate in their home country, as visa regulations can change quickly.
US citizens that need to renew, add pages, change name, or just get a new US passport, can Apply for a New US Passport here.