African-American History Online:

Features thousands of cross-referenced entries, covering the entire spectrum of African-American history over the past 500 years.

Ancient and Medieval History Online:

Focuses on Egypt, Mesopotamia, Greece, Rome, Africa, Europe, the Americas, Asia, and Islamic Empire from prehistory through the 1500s.

American History Online:

Spans more than 500 years of political, military, social, and cultural history to cover the American experience.

Modern World History Online:

Covers the people, places, and events in the broad expanse of history - from the mid-15th century to the present.

 Selecting History will reveal many reference books on a variety of topics.  One can search within these printed books