Edwin M. Knowles owned a huge pottery factory that was in operation from 1900 - 1963. Knowles produced dishware that featured floral themes, Art Deco themes, plain whiteware, and other simple designs. They closed due competition from cheap imported goods. A second Knowles company, operated buy a business that bought the Knowles name made collectible plates featuring scenes from classic films in the 1980s and 1990s. Your semi-vitreous dishware was from the earlier version.
Search for your pattern online. If you don't know the name of the pattern, you can find lists of Knowles patterns online at Robins Nest or Laurelhollowpark. You can see what the patterns look like by searching on Google image.
There is a lot of Knowles dishware for sale online. Prices vary widely. Check for sold prices on your particular pattern by clicking Advanced search on eBay. Value is only as good as what people are willing to pay. What people ask may not reflect an actual value.