This series of handbooks would be a good place for information on themes and thinkers.
You can search within the set, best to use the Books tab instead of the Articles.
Use WorldCat UMD (the catalog) to locate books on liberty political theory.
Some sample searches:
"positive liberty"
"negative liberty"
"liberty philosophy"
liberty philosophy (without quotes which broadens the results)
su:liberty (limit search to the subject)
multiculturalism immigration
refugee* cosmopolitanism