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 (the catalog) to locate ebooks on political theory in general and also specific theories.
Use the Advanced Search and limit to ebooks
View eBook link goes to HathiTrust? - please see this FAQ for details on how to access:
https://umd.libanswers.com/faq/296480