This site offers links to a lot of information about Islamic architecture, including articles on both specific buildings and on general topics. For example, the article on mosques describes the history, functions, and uses of mosques, and gives links to many specific mosques in different parts of the Islamic world
Particularly important mosques to look at would include:
- the Masjid al-Haram in Mecca, Saudi Arabia
- the Masjid al-Nabawi in Medina, Saudi Arabia
- the Al-Aqsa Mosque in Jerusalem
- the Great Mosque in Damascus, Syria
- the Mezquita in Cordoba, Spain
- the Djenne Mosque in Timbuktu, Mali
- the Maydan-i-Shah in Isfahan, Iran
The article on Mosques at Wikipedia is also excellent, as is its article on Islamic Architecture
A couple of the most important Islamic buildings that are not mosques:
- the Dome of the Rock in Jerusalem
- the Taj Mahal in Agra, India
