Can't help with the bookends, but there is summary info on St Clair glassworks on the Glass Encyclopedia page at: http://www.glassencyclopedia.com/StClairglass.html
The paperweights of the St Clair family follow a long tradition of American designs incorporating swirls and bubbles. Most are marked to the base with an impressed logo giving the name of the particular company / maker such as "Joe St Clair, "Maude & Bob St Clair".
If your weight has no impressed logo then that may tie in with it being a "seconds" piece (or "mistake" as suggested). Even if there is a logo, it could still be a standard production piece that went wrong and for some reason was passed to your family instead of being rejected.
A search in eBay lists will usually show a few typical St Clair weights and these are also often listed in various American paperweight dealer sites.