If you go for a book on dumps, I recommend Mary Skotnicki's as by far the best - she has probably the best collection of dumps in the world, and the book shows some very unusual pieces, as well as the more common place (she used to fight Bill Gaskill for anything rare...). It does not give current prices, of course - which are perhaps a third or less of what they were 10 years ago for the run of the mill designs.
Many of the sulphide dumps, foil dumps and 'bubble' dumps are mid to late Victorian, and from many glass factories in the UK. However, they were made through to the 1960s in at least one factory. The hardest to find are signed 'Kilner' or 'Redfearn' with an impressed seal on the base.
Beware of very clean, relatively tall pieces in pale blue-green glass, with tiers of flowers and with a flat ground base. These came in by the crate load a few years ago, all the way from China, and were offered for sale as 'Victorian dumps' at ludicrously high prices by a number of sellers on eBay and at Antiques Fairs. Quite a few people were taken in...but thankfully they seem to have all been sold now.