Best two options for secondary historical sources:
BOOKS and NEWSPAPERS
Books are written about historical events (such as Hurricane Katrina) based on research done in primary sources -- interviews, video footage, diaries (which could be blog posts), government documents, etc. They are secondary, but that doesn't mean they are less valuable than primary source material. Good history books gather and assess and interpret events on the basis of thorough research.
Newspapers can function as both primary and secondary sources. Newspaper reporting that supplies eyewitness coverage of events by journalists would be primary source sort of material. But newspapers also will function like historical books, as secondary sources, when they gather and interpret and analyze events -- such as the impact of the hurricane on the lives of people on the Gulf Coast.