This is, of course, a research project but you'll still need historical context for your projects.
Better still, other scholars who have studied this topic will help you determine where to look for primary sources -- they will highlight local collections, federal records, scanned materials, etc.
To start, we would begin with a pretty basic search in our catalog for Manassas AND history. The majority of books in our collection are about Bull Run specifically, but there are a few general books of some interest. A better search is perhaps "Prince William County" AND history.
We don't typically collect local histories, but there are some promising works in WorldCat that we can order via ILL at no cost.
This includes 25 biographies. The majority of books in WorldCat specifically about Manassas are about the battlefield.
Without specific topics, it's hard to narrow down into article searches just yet, but general searches in America: History and life may be useful.
Example searches:
SU (manassas OR "prince william county") [limited to articles focused on pre 1860
SU (manassas OR "prince william county") no date limiters
For thoroughness, a complete listing of articles in All our EBSCO databases
Not much here, but I tried a search for subject((manassas OR "prince william county") AND history) and there are a couple interesting results