There is a book which I can highly recommend that has loads about John, Gertrude and Peter Gentile and the Gentil glass co. Also a load of other makers of that era - such as Hamon, Hacker, Larson, Degenhart, Hansen, Iorio, Kaziun to name but a few.
It title is "American Glass Paperweights and their makers" written by Jean Melvin.
It is OOP but there are plenty of used copies if you check abebooks.co.uk Make sure you go for the revised edtion - 280 pages so a good read !
Suggest you do as I did - get one from the US and get them to ship it in a global express envelope which in my experience is just as quick as getting Royal Mail to send it inland and only costs $9.50!!
There are currently 5 copies listed in the US at £10 or less. Cheapest one located in the UK is £40 !!
Hope this helps