A digital library containing some of the printed primary and secondary sources for the history of the British Isles. This link will take you to a list where you can choose from the reign of Charles I and Interregnum (1649-60), which covers the execution of Charles I to the Restoration of the monarchy.
The British Civil Wars, Commonwealth and Protectorate 1638-1660
A very comprehensive site which is still being expanded. Check out the SiteMap there to see just how much is covered. Highly recommended.
Source analysis activities by the National Archives, UK. Topics include: Why did people go to war in 1642? Did people want the king back in 1646? Why did Britain become a republic? and What kind of ruler was Cromwell?