This is NOT Alexandrite. It may well be by Thomas Webb, but it does not have the correct colouring - as Frank suggested.
There were 10 pieces in the two Michael Parkington sales back in 1997/98 - I handled all of them (Pt I - lots 281, 2 & 3; Pt II - lots 302, 03 & 04) I have owned three pieces, and have been lucky enough to handle others.
The correct colouring is: rising from citrine, going into rose pink, then into blue and, finally a chocolate brown rim. Most pieces are thin, which would probably make them unsuitable for such use. This would also suggest that the colouring on the night light is "up-side-down", since the rose pink is within the body and the citrine toward the rim. There appears to be no blue and it is doubtful that there is a chocolate brown rim from what can be seen in the photo.
True Thomas Webb Alexandrite commands substantual prices, so I don't think it is advisable to assume it to be that rare commodity. If the colouring for all four constituants is poor, or the chocolate brown is missing, then the value is commensurately (even substantually) less.
As for reference sources, I'm afraid little has been written about the subject. Yes, Hajdamach "British Glass", also, earlier, and with less accurate information, "Decorative Victorian Glass" by Cyril Manley - otherwise I cannot recall any other source at present.